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Author
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Topic: Back to the Topo II - where does it end?
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Pim van Meurs
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Member # 541
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posted 03. May 2003 19:08
Nelson, let me ask you a question. Did NaeI function as a topoisomerase before the mutation? What about afterwards?
You yourself state that "What the paper notes is that NaeI endonuclease is changed to a "stand alone" topoisomerase/recombinase "
So i san endonuclease the same as a topoisomerase/recombinase ?
As far as your 'hypothesis' is concerned, it seems that while it may be a hypothesis there is nothing in it which makes it an ID hypothesis. I was hoping for a hypothesis directly relevant to ID.
From the paper
quote:
Changing Leu43 to lysine converts NaeI from endonuclease to topoisomerase/ recombinase, suggesting that NaeI is a bridge between these protein families2.
See "Huai Q, Colandene JD, Chen YQ, et al., Crystal structure of NaeI - an evolutionary bridge between DNA endonuclease and topoisomerase. EMBO J 19: (12) 3110-3118 JUN 15 2000"
"NAE:I may be an evolutionary bridge between endonuclease and DNA processing enzymes."
Or as Hengming Ke, PhD states it
"NaeI is a type IIe DNA restriction endonuclease. It has significant homology to the active site of DNA ligase. A single L43K mutation transforms the nuclease activity of NaeI into a novel site-specific DNA topoisomerase. Our crystal structure reveals that NaeI contains two structural domains: endonuclease and topoisomerase domains that may respectively correspond to the endonuclease and topoisomerase activity of NaeI. Therefore, NaeI is an evolutionary bridge between endonucleases and the DNA processing proteins such as topoisomerases. Our study also implies that DNA binding proteins, although they are involved in various biological processes, may evolve from a few common ancestors. "
Woese comments on the universal ancestor by stating that "A linear (small) chromosome makes both replication and transcription simple from a topological perspective (topoisomerases don’t seem to be needed)."
As well as
"This finding supports the idea that the DNA topoisomerases evolved to solve the topological problems of DNA as it became progressively longer or when ring-shaped DNA emerged; for short linear chromosomes, the topological problems can be alleviated by movement of their ends."
James Wang "Cellular roles of DNA topoisomerases: a molecular perspective" Nat. Rev. Mol. Cell Biol. 3(6):430-40.
[ 03. May 2003, 20:34: Message edited by: Pim van Meurs ]
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Nel
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posted 04. May 2003 14:28
Pim,
What the mutation "unpacked" was pre-existing topo I activity. Think of the amino acid at position 43 as a switch turning on and off the activity (going from an endonuclease to a topo I). This is not a mutation "creating" a topo I.
The hypothesis that I point out is directly relevant to ID, since you yourself quote a paper which states:
quote:
"This finding supports the idea that the DNA topoisomerases evolved to solve the topological problems of DNA as it became progressively longer or when ring-shaped DNA emerged; for short linear chromosomes, the topological problems can be alleviated by movement of their ends."
The hypothesis that ID might propose is that DNA topoisomerases were already functioning in the first "seeded" organisms and that organisms with short linear chromosomes never existed, they did not pre-exist present organisms. Can you tell me how this is not relevant to ID?
[ 04. May 2003, 14:28: Message edited by: Nelson_Alonso ]
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Pim van Meurs
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posted 04. May 2003 14:46
Nelson: What the mutation "unpacked" was pre-existing topo I activity. Think of the amino acid at position 43 as a switch turning on and off the activity (going from an endonuclease to a topo I). This is not a mutation "creating" a topo I.
Fascinating. Pre-existing topo activity that only required a single mutation to be turned on. Sounds like a good example of evolution to me.
[added in edit] Let me explain, the idea is that NaeI may be an evolutionary bridge between endonuclease and topoisomerase. A possible pathway would be that endonuclease gene duplicated and one of them underwent a single point mutations. Voila.
Nelson: The hypothesis that ID might propose is that DNA topoisomerases were already functioning in the first "seeded" organisms and that organisms with short linear chromosomes never existed, they did not pre-exist present organisms. Can you tell me how this is not relevant to ID?
What relevance does this have to infer intelligent design? I see a fully natural hypothesis and no evidence of the 'seeded' part. Please show how ID would go about to support such a hypothesis and why such a hypothesis should be seen as supporting ID? Surely the assumption of 'seeded' may or may not require intelligent design so we just have moved the ID inference a bit further back.
[ 04. May 2003, 14:58: Message edited by: Pim van Meurs ]
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Nel
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posted 04. May 2003 15:44
Pim,
Well actually the paper points to multiple ancestors or a more complex common ancestor, however, the topoisomerase is not "created" by a single mutation, as I keep pointing out, which is why the activity may come from both the N-terminus and C-terminus.
How is the pre-existence of an organism with a complex topo II and replication system with no ancestor that was seeded here on earth by the thing that designed the organism not relevant to intelligent design? How does the assumption of purposeful seeding not require intelligent design?
[ 04. May 2003, 15:45: Message edited by: Nelson_Alonso ]
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yersinia
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posted 04. May 2003 16:21
quote:
Pim,
Well actually the paper points to multiple ancestors or a more complex common ancestor, however, the topoisomerase is not "created" by a single mutation, as I keep pointing out, which is why the activity may come from both the N-terminus and C-terminus.
How is the pre-existence of an organism with a complex topo II and replication system with no ancestor that was seeded here on earth by the thing that designed the organism not relevant to intelligent design? How does the assumption of purposeful seeding not require intelligent design?
Nelson, you are the most confusing writer sometimes...
According to evolutionary theory, everything new is a modified version of something old. Very occasionally a new gene may get produced from "nothing" -- a frameshift mutation of junk DNA or something -- but this is relatively rare. We *do not expect* **anything** to be "created" from nothing, like you keep supposing for polymerases, the flagellum, etc. **Any** calculations or evaluations of evolution based on "creation from nothing" are not evaluating the actual theory.
That endonuclease is obviously a perfectly functional endonuclease without unnecessary parts (they would have been lost long ago if unnecessary). To suppose that a "front-loaded" polymerase was also encoded billions of years ago, and although never used was discovered in the lab, is highly extravagant. It is much simpler to postulate, as the authors do, that:
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Therefore, NaeI is an evolutionary bridge between endonucleases and the DNA processing proteins such as topoisomerases. Our study also implies that DNA binding proteins, although they are involved in various biological processes, may evolve from a few common ancestors.
In short, even when we have knock-down evidence that the evolutionary transition in question is easy, Nelson refuses to believe it. The only thing that would convince him is a combination of a time machine and a 21st century biochem lab that went back in time to observe every single transition and precursor organism in excrutiating detail.
But this kind of requirement is not a reasonable standard for reaching conclusions in science, is it?
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Pim van Meurs
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posted 04. May 2003 16:29
Nelson: How does the assumption of purposeful seeding not require intelligent design?
Note that Nelson did not initially state "purposeful seeding" but merely that "The hypothesis that ID might propose is that DNA topoisomerases were already functioning in the first "seeded" organisms". If Nelson now wants to add purposeful seeding then we have even more complications since now the ID inference which has been moved back to the instance of the 'seeding' will not only have to be shown to be not explainable by chance or regularity hypotheses but also to be purposeful which is a much more stringent requirement than even Demsbki has formulated for his design inference. One may very well work from that presumption but that does not make the presumption necessarily true or even relevant to the resulting hypothesis.
Nelson also argues that "... however, the topoisomerase is not "created" by a single mutation, as I keep pointing out, which is why the activity may come from both the N-terminus and C-terminus."
The topoisomerase activity is created by a single mutation, before the mutation NaeI performs as an endonuclease, afterward it works as a topoisomerase. Perhaps Nelson wants to argue that many of the required sequences were already present in NaeI performing as a endonuclease and thus that more than a single mutation was required to get to NaeI performing as such but from an evolutionary perspective that is largely irrelevant. What is relevant is that a single mutation can change NaeI from a functional endonuclease to a functional topoisomerase, indicating that NaeI can bridge the evolutionary gap between the two domains. In fact it shows that there is not even the need for functional intermediates. A great example of evolution in which a single point mutation can bridge the gap.
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yersinia
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posted 04. May 2003 16:32
Regarding the flagellum, Nelson, why do your confident pronouncements leave out things like:
- the MotAB homologies to nonflagellar proton-driven membrane proteins
- the homology between FliI and F1F0 ATPase [edit: Nelson points out that this is unclear, I am referring to the homology between FliI and the beta subunit of the F1 part of the F1F0 ATPase, as I clarified in the next post; as I say below the alpha and beta subunits are considered homologs of each other]
- the possibility that the four rod proteins, the hook and filament proteins are all modified copies of each other
- the fact that gram-positive bacteria don't need the outer ring proteins, and that these proteins when they are used are secreted by the type II pathway, indicating that they may be a modified secretin protein (and the ring proteins are in fact replaced by a secretin pore in the virulence system)
- the fact that bacteria flagella serve a double purpose as adhesion organelles, which of course does not require a functional motor
- the fact that several other bacterial motility systems, including the archaeal flagellum, also have homologs to nonflagellar transport systems, usually phylogenetically basal
- that the flagellum chemotaxis proteins have homologs with a large number of nonflagellar signalling proteins
- and, of course, that membrane transportation can clearly be done by simpler systems than the (*nine* parts I think) T3SS, because we have numerous examples of membrane transporters and pores going all the way down to single-protein pores.
Until you and Dembski take all of these things into account in your calculations, I'm not sure why we should even begin to take your numbers seriously.
[ 06. May 2003, 16:00: Message edited by: yersinia ]
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Nel
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posted 05. May 2003 15:14
Yersinia and Pim,
Regarding NaeI being changed to a topo I, this is not a change from a few random mutations brought about by natural selection. Again there is no evidence of an organism able to carry out the feat of replication without topoisomerase. This is not a creation of a topo I from a less complex precursor, which is why I say it's irrelevant to the ability of natural selection. You can hypothesize that an endonuclease, once formed, can be easily changed to a topo I and that things evolved from there. But this would require a precise, single, point mutation at position 43. What this shows us is that both an endonuclease and a topo I might be IC, in that a single change cannot preserve function. However, the activity of topo I may require both domains to jointly accomplish the topoisomerization and so it is important to realize that topo I function was not "created" by a single mutation. Now, the only reason why the researches presume it is an "evolutionary bridge" is based on the fact that they co-exist in the same molecule. I do not think it is knock-down evidence by a long shot. None of these thigns bears any kind of resemblance to eachother. There is absolutley no sequence homology to any known topoisomerase. This is a fascinating example of a dual-function protein but hypothesis about it can go either way, in my opinion.
To Pim,
As far as my hypothesis about the first organisms already having a complex replication system which requires topo II, this is not dependant on the fact that the seeding on this earth was purposeful. The designer could have accidently left this consortium of cells here or he could have done it on purpose, nonetheless, the evidence shows, in my opinion that there was probably no simpler version that natural selection and random mutation was capable of building. This is not explainable by chance and regularity unless you are saying that chance and regularity poofed the more complex version into existence.
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yersinia
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posted 05. May 2003 16:03
quote:
This is not a creation of a topo I from a less complex precursor, which is why I say it's irrelevant to the ability of natural selection. You can hypothesize that an endonuclease, once formed, can be easily changed to a topo I and that things evolved from there. But this would require a precise, single, point mutation at position 43. What this shows us is that both an endonuclease and a topo I might be IC, in that a single change cannot preserve function.
This is completely incoherant. What's unlikely about a single point mutation? How can a *demonstrated* change from one function to another function show that "a single change cannot preserve function"? How can a single enzyme be IC anyway?
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Nel
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posted 05. May 2003 16:18
Nic,
This is way off topic but you've made a number of assertions that needs to be addressed. For example, you say that I leave out various homologies. But in fact, I don't, the homologies you cite strengthen the fact that there exist mini-IC systems that would require unselectable steps for any evolutionary pathway. For example, with regard to MotAB, the motor complex in the flagellum requires 3 components, fliG, motA, and motB, and this is analogous to the ICness of ExbB, ExbD, and TonB. Still, there are more problems with regard to the logistical operations needed to be performed by a stochastic process in order to make this thing work from an ion channel, as Mike points out:
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Of all the ways to mutate an ion channel, the number of ways that would result in its interacting with the base of some filament is surely in the distinct minority. And of all the ways to mutate an ion channel that gloms onto a filament, the number of ways to mutate it such that rotation does not occur is probably much higher than the number of ways to elicit some rotation...This [mutation] allows some ion channel to glom onto the base of a filament and open its channel and expose the ion flow to the proto-rotor in such a way that a set of electrostatic interactions just happen to form and elicit significant rotation. Suffice it to say that such an improbable mutation has never been observed in nature or the lab.
The homology between FliI is said to only be homologous to the b subunit of the F-ATP synthase, not the whole 8 parts of the synthase, the whole thing requires all 8 parts to work.
With respect to the possibility that the rod proteins were derived from eachother, Mike Gene addresses this in his essay:
quote:
It would seem there is no reason why the rod should be built around three proteins instead of simply one. Yet these three gene products are found in all flagella, dating back to the putative ancestral flagellum. This suggests one protein is not sufficient to form a functioning flagellar rod. Furthermore, the size of these proteins among these five distantly related bacteria has been held relatively constant (Fig 2), despite billions of years of experiencing very different selective pressures. It would seem some form of constraint or specification is at work, as natural selection will not tolerate too much deviation. And these size constraints map back to the last common ancestral flagellum, indistinguishable from the first flagellum.
Gram positive bacteria don't need the L and P rings because they simply do not have an outer membrane. I think that it's as simple as that (well not really, Mike has hinted at how this can illuminate something about it's origin but hasn't discussed this yet).
I'm not familiar with a flagellum serving as an adhesion organelle, although I'm familiar with pili that do. With that you continue to introduce more unselectable steps, the irreducible complexity of the folding of P Pilus, not to mention the sophisticated mechanisms, donor strand exchange and donor strand complementation. The pilus itself is made up of 5 parts, PapK PapA,PapE,PapK, and PapG. Furthermore, the pilus doesn't seem to be able to secrete proteins, and the biggest difference between flagella and pili is that flagella are built from the top to the bottom, whereas pili are built from the bottom to the top. The notion of a simple filament sticking to an export machine seems to vanish.
As far as the last two, I would need to see those for myself. I am cautious at Nic's constant mentioning of homology due to the fact that he doesn't take anything like convergence or coincidence (or common design) into account, which kind of makes me careful to accept his criteria for saying something is homologous. And as we have seen, structural homologies are not good indicators of common descent.
As Mike Gene states, as far as pores go, not any old pore would do. The logic of this is that with all the pores that exist, if things were that simple, we should see plasticity among flagella of eubacteria (indeed this is one of the major problems with co-option stories, no plasticity.)
To bring this discussion back to the topic of this thread, it is interesting to note that, with respect to the three kingdoms, there is a growing sense of the consortium of cells theory put forth by these type of investigations (Mike Gene is the only who predicted this from an ID perspective).
quote:
In a June 1998 issue of the Proceedings of the National Academy of Sciences, Woese proposed a theory of the universal ancestor, based on a genetic annealing model in which lateral gene transfer played a major role. He wrote: "The universal ancestor is not a discrete entity. It is, rather, a diverse community of cells that survives and evolves as a biological unit. This communal ancestor has a physical history but not a genealogical one. Over time, this ancestor refined into a smaller number of increasingly complex cell types with the ancestors of the three primary groupings of organisms arising as a result."
In the same journal in June 2002, Woese refined his theory, arguing that life did not begin with one primordial cell. Instead, he said there were initially at least three simple types of loosely constructed cellular organizations swimming in a pool of genes, evolving in a communal way that aided one another in bootstrapping into the three distinct types of cells by sharing through lateral gene transfer their evolutionary inventions. source
That may or may not be exactly what Gene has in mind, but I think it's pretty damn close.
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Nel
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posted 05. May 2003 16:22
Nic,
It was demonstrated in the Huang paper. A single amino acid change disrupts the function of a type II restriction endonuclease and makes it act as a topo I. However, the fact that the topo activity may require both the N and C terminus shows that this is not brought about by a single mutation, but the mutation itself "unpacked" pre-existing activity.
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yersinia
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posted 05. May 2003 16:59
Similarly, mutations that confer resistance to antibiotics are only "unpacking" pre-existing capability, eh? This is a very convoluted way to think about things.
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yersinia
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posted 05. May 2003 18:49
quote:
Nic,
This is way off topic but you've made a number of assertions that needs to be addressed. For example, you say that I leave out various homologies. But in fact, I don't, the homologies you cite strengthen the fact that there exist mini-IC systems that would require unselectable steps for any evolutionary pathway.
"Mini-IC systems?" But I thought that IC meant that the *whole thing* was IC?
quote:
For example, with regard to MotAB, the motor complex in the flagellum requires 3 components, fliG, motA, and motB, and this is analogous to the ICness of ExbB, ExbD, and TonB.
ExbB and ExbD act on TonB, IIRC there's another ExbBD pair homolog that acts on something else. And motAB act on fliG in a similar fashion. Which is why Kojima & Blair wrote in Biochemistry (2001, vol. 40, pp. 13041-50),
quote:
The occurrence of significant conformational change in the stator has implications not only for the present-day mechanism but also for the evolution of the flagellar motor. A membrane complex that undergoes proton-driven conformational changes could perform useful work in contexts other than (and simpler than) the flagellar motor, and ancestral forms of the MotA/MotB complex might have arisen independently of any part of the rotor.
Nelson continues,
quote:
Still, there are more problems with regard to the logistical operations needed to be performed by a stochastic process in order to make this thing work from an ion channel, as Mike points out:
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--------------------------------------------------
Of all the ways to mutate an ion channel, the number of ways that would result in its interacting with the base of some filament is surely in the distinct minority. And of all the ways to mutate an ion channel that gloms onto a filament, the number of ways to mutate it such that rotation does not occur is probably much higher than the number of ways to elicit some rotation...
None of this is disputed, but all it is saying is that "beneficial mutations are unlikely" which is no surprise to evolutionary biology. All that is needed is one rare beneficial mutation amongst millions of ones that "don't work" and selection will pick it out. And, FWIW, mutation experiments seem to indicate that there is a fair bit of flexibility where motor function is retained despite mutation in the MotB--rotor interface.
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This [mutation] allows some ion channel to glom onto the base of a filament and open its channel and expose the ion flow to the proto-rotor in such a way that a set of electrostatic interactions just happen to form and elicit significant rotation.
This depends upon a particular model of the mechanism of flagellar rotation, which is an unsolved question (Hey! A place for the IDer to act in modern times! Unless you're a nasty methodological naturalist...). Based on the ExbBD homology, I would expect that the ion channel is essentially internal to the ExbBD system and that the energy resulting from H+ flow is transferred through the protein structure via conformational change to act on the flagellum base (or on TolB etc.) to do work at a distance. Call it a prediction if you like. The electrostatic model certainly is elegant, but based on ExbBD homologs being independent units doing "work at a distance" in several different systems, it seems unlikely. Time will tell.
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The homology between FliI is said to only be homologous to the b subunit of the F-ATP synthase, not the whole 8 parts of the synthase, the whole thing requires all 8 parts to work.
IIRC the 3 alpha and 3 beta subunits of the F1 ATPase are thought to be homologs of each other, with only the beta subunits retaining ATPase activity:
The similarity of the beta subunit to FliI is something like 33% which is highly statistically significant, well above the ambiguous level:
quote:
How might these ATPases catalyze processive protein export? Spa47 (the Shigella FliI homolog) shares 33% amino acid identity with the beta-subunit of F1-ATPase. Proteins with >30% sequence identity have a high probability of sharing similar structures (69). Active F1-ATPase is a heterohexamer consisting of alternating alpha- and beta-subunits with a gamma-subunit inserted in a central channel where it rotates during the catalytic cycle (70). No equivalent of the alpha-subunit of F1-ATPases is found within flagellar or TTSS-encoding operons, so we assume that the type III export motor is a homohexamer. When modeled on the F1 structure, Spa47 fits at the inner membrane base of our NC structure (Fig. 3). It would contain a central channel aligned with the one found within the NC and of similar diameter to it, through which the proteins could be secreted (see Supporting Text).
Source: Blocker A, Komoriya K, Aizawa S. Proc Natl Acad Sci U S A 2003 Mar 18;100(6):3027-30. Type III secretion systems and bacterial flagella: Insights into their function from structural similarities. http://www.pnas.org/cgi/content/full/100/6/3027
...the assumption of homology seems to be confirmed by the fact that the resulting protein complex fits well into their model of T3SS structure.
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With respect to the possibility that the rod proteins were derived from each other, Mike Gene addresses this in his essay:
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It would seem there is no reason why the rod should be built around three proteins instead of simply one. Yet these three gene products are found in all flagella, dating back to the putative ancestral flagellum. This suggests one protein is not sufficient to form a functioning flagellar rod. Furthermore, the size of these proteins among these five distantly related bacteria has been held relatively constant (Fig 2), despite billions of years of experiencing very different selective pressures. It would seem some form of constraint or specification is at work, as natural selection will not tolerate too much deviation. And these size constraints map back to the last common ancestral flagellum, indistinguishable from the first flagellum.
It seems highly unlikely that the different rod proteins have radically different functions. Probably their retention has to do with starting and stopping the rod construction process, wherein it would be helpful to have a tightly-controlled starting and stopping points, but where simpler mechanisms could suffice at first, e.g. just generating a certain amount of one rod protein.
All that is required to get from one rod protein to several is the sub-functionalization of gene copies, which you, Nelson, have enthusiastically endorsed in other threads.
quote:
Gram positive bacteria don't need the L and P rings because they simply do not have an outer membrane. I think that it's as simple as that (well not really, Mike has hinted at how this can illuminate something about it's origin but hasn't discussed this yet).
So, are they part of the IC system or not, and how many orders of magnitude difference in probability does this decision result in?
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I'm not familiar with a flagellum serving as an adhesion organelle, although I'm familiar with pili that do.
An example (one of many) was cited in the original immune system thread.
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With that you continue to introduce more unselectable steps, the irreducible complexity of the folding of P Pilus, not to mention the sophisticated mechanisms, donor strand exchange and donor strand complementation. The pilus itself is made up of 5 parts, PapK PapA,PapE,PapK, and PapG. Furthermore, the pilus doesn't seem to be able to secrete proteins, and the biggest difference between flagella and pili is that flagella are built from the top to the bottom, whereas pili are built from the bottom to the top. The notion of a simple filament sticking to an export machine seems to vanish.
Huh? There are many kinds of pili, the term basically means "sticky-outy bit" as far as I can tell. And it seems that basically every transport system that has been identified has versions that support extracellular extensions. They all have to secrete proteins, in order to get the pilus proteins to the outside. You are focusing on the abilities of the P pilus, built on a Type I transporter IIRC, but we are talking about type III secretion systems. It appears that flagella and pili can be built from the top or the bottom, since Type III and Type IV (both have motile flagella and nonmotile pili systems) do each respectively.
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As far as the last two, I would need to see those for myself. I am cautious at Nic's constant mentioning of homology due to the fact that he doesn't take anything like convergence or coincidence (or common design) into account, which kind of makes me careful to accept his criteria for saying something is homologous. And as we have seen, structural homologies are not good indicators of common descent.
Sequence and structural homology are well-documented concepts with a great deal of predictive theory and evidence behind them. "Common design" is the at-random invocation of "the designer would have put statistically significant but functionally pointless similarities in a phylogenetic pattern for no good reason" which is erected as a smoke screen to avoid dealing with the statistically significant but functionally unnecessary similarity.
For the chemotaxis homologs, see this thread. The similarites are in sequence, not just structure IIRC.
As for the secretin, the evidence is merely suggestive at this point: there is no statistical evidence of homology at this point AFAIK. But it is interesting that the outer membrane ring of flagella is secreted by a type II rather than type III pathway, as is the secretin used in the outer membrane of type III virulence systems, as is the outer membrane secretin used in a large number of other transport systems (see: Bitter W., Secretins of Pseudomonas aeruginosa: large holes in the outer membrane. Arch Microbiol 2003 May;179(5):307-14). If structural homology is discovered at some point, then another piece of the puzzle will have fallen into place.
Even here we are a far cry from Dembski's strawman of getting flagellum parts from the mythical, mystical "protein supermarket".
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As Mike Gene states, as far as pores go, not any old pore would do. The logic of this is that with all the pores that exist, if things were that simple, we should see plasticity among flagella of eubacteria (indeed this is one of the major problems with co-option stories, no plasticity.)
I'm not sure what the logic is. The point is not plasticity in eubacterial flagella (although there is some of that), the point should be that several different complex motility systems are based on several different transport systems. Assuming that the eubacterial flagella was "the goal" is unwarranted (a form of "painting the target around the arrow"), because we have archaeal flagella, twitching motility, slime-secretion motility, and probably other systems. It appears that something like half of the classes of secretion systems have the capability to be modified to make use of their "sticky-outy bits" for motility. Evolutionary theory doesn't need to postulate that "any old pore" would do, just that there are a lot of different pores, and that some of them would do.
Woese is a different topic so I won't address that.
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Pim van Meurs
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posted 06. May 2003 00:42
Nic are you familiar with this article
Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria
and This one
ASSEMBLY AND FUNCTION OF TYPE III SECRETORY SYSTEMS Guy R. Cornelis and Fred´erique Van Gijsegem
[ 06. May 2003, 00:48: Message edited by: Pim van Meurs ]
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yersinia
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posted 06. May 2003 02:54
Hi Pim,
The link on the first article is a little messed up. It reviews the Type I-IV secretion systems briefly but I recently read another reference that listed several more.
The second one I had seen before, it describes the Type III secretion system provocatively as "a complex weapon for close combat", responsible for a large number of nasty diseases -- bubonic plague, salmonella, chlamydia, pertussis...makes you wonder about the designer of that clever system, for sure.
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