posted 29. December 2003 19:09                   

quote:

---

J Mol Evol. 1991 Oct;33(4):367-78. Related Articles, Links

The evolution of rhodopsins and neurotransmitter receptors.

Fryxell KJ, Meyerowitz EM.

Department of Biology, University of California, Riverside 92521.

Rhodopsins share a limited number of amino acid identities with a variety of other integral membrane proteins. Most of these proteins have seven putative transmembrane segments and are likely to play a role in transmembrane signaling. We have undertaken a systematic series of comparisons of primary and secondary structure in order to clarify the functional and evolutionary significance of these sequence similarities. On the basis of consistently high similarity scores, we find that the most internally consistent definition of the rhodopsin gene family would include vertebrate rhodopsins, alpha- and beta-adrenergic receptors, M1 and M2 muscarinic acetylcholine receptors, substance K receptors, and insect rhodopsins, while excluding bacteriorhodopsin, the mass human oncogene, vertebrate and insect nicotinic acetylcholine receptors, and the yeast STE2 and STE3 peptide receptors. The rhodopsin gene family is highly diverged at the primary sequence level but has maintained a conserved secondary structure, including a previously unidentified hierarchy of transmembrane segment hydrophobicity. We have developed new computer algorithms for progressive multiple sequence alignment and the analysis of local conservation of protein domains, and we have used these algorithms to examine the phylogeny of the rhodopsin gene family and the changing domains of sequence conservation. The results show striking differences and similarities in the conserved domains in each of the three main branches of the rhodopsin gene family, and indicate that color vision arose independently in the lines of descent leading to modern humans and fruit flies.

---

quote:

---

Volume 1 Issue 9 Page 787 - September 1996


Molecular evolution of retinal and nonretinal opsins
Shozo Yokoyama

Vision and the circadian rhythms of various biological functions are triggered by phototransduction. The retinal and nonretinal (pineal gland-specific) opsins are traced back to a single common ancestor. Evolutionary analyses of these opsins identify amino acid changes that are potentially important in the regulation of wavelength absorption of photosensitive moleculesvisual pigments. Such theoretical predictions can now be tested experimentally using site-directed mutagenesis; expressing the mutagenized opsins in tissue culture cells, reconstituting with 11-cis retinal, and measuring the absorption spectra of the regenerated visual pigments.

---

quote:

---

Conclusions. These results support the hypothesis that the disparate families of GPCRs possess modular components, comprised of TMSs, at least some of which share a common ancestry, demonstrated by multiple statistically significant alignments. This approach may be universally applicable to the elucidation of distant relationships among other families of polytopic membrane proteins.

---

quote:

---

J Mol Evol. 1993 Jan;36(1):79-95. Related Articles, Links

The evolution of proteins from random amino acid sequences. I. Evidence from the lengthwise distribution of amino acids in modern protein sequences.

White SH, Jacobs RE.

Department of Physiology and Biophysics, University of California, Irvine 92717.

We examine in this paper one of the expected consequences of the hypothesis that modern proteins evolved from random heteropeptide sequences. Specifically, we investigate the lengthwise distributions of amino acids in a set of 1,789 protein sequences with little sequence identify using the run test statistic (ro) of Mood (1940, Ann. Math. Stat. 11, 367-392). The probability density of ro for a collection of random sequences has mean = 0 and variance = 1 [the N(0,1) distribution] and can be used to measure the tendency of amino acids of a given type to cluster together in a sequence relative to that of a random sequence. We implement the run test using binary representations of protein sequences in which the amino acids of interest are assigned a value of 1 and all others a value of 0. We consider individual amino acids and sets of various combinations of them based upon hydrophobicity (4 sets), charge (3 sets), volume (4 sets), and secondary structure propensity (3 sets). We find that any sequence chosen randomly has a 90% or greater chance of having a lengthwise distribution of amino acids that is indistinguishable from the random expectation regardless of amino acid type. We regard this as strong support for the random-origin hypothesis. However, we do observe significant deviations from the random expectation as might be expected after billions years of evolution. Two important global trends are found: (1) Amino acids with a strong alpha-helix propensity show a strong tendency to cluster whereas those with beta-sheet or reverse-turn propensity do not. (2) Clustered rather than evenly distributed patterns tend to be preferred by the individual amino acids and this is particularly so for methionine. Finally, we consider the problem of reconciling the random nature of protein sequences with structurally meaningful periodic "patterns" that can be detected by sliding-window, autocorrelation, and Fourier analyses. Two examples, rhodopsin and bacteriorhodopsin, show that such patterns are a natural feature of random sequences.

---

posted 29. December 2003 21:48                   

quote:

---

On the basis of consistently high similarity scores, we find that the most internally consistent definition of the rhodopsin gene family would include vertebrate rhodopsins, alpha- and beta-adrenergic receptors, M1 and M2 muscarinic acetylcholine receptors, substance K receptors, and insect rhodopsins, while excluding bacteriorhodopsin, the mass human oncogene, vertebrate and insect nicotinic acetylcholine receptors, and the yeast STE2 and STE3 peptide receptors.

---

quote:

---

Comparisons of the bacteriorhodopsins with other families did not yield significant results. This result suggests that either vast evolutionary distances have caused the TMSs to diverge past the point of recognition, or that GPCRs and bacteriorhodopsins have independent origins as polytopic membrane proteins.

---

posted 29. December 2003 22:10                   

quote:

---

It seems to me that "the dirty little secret" of all this "convergence" is that it calls into question a basic assumption of evolution in the first place. If so many structures with such a high degree of similarity were not derived from common ancestry, then how reliable are our assumptions about common ancestry in other cases?

---

What is really interesting is that these kind of examples point to "common design". These examples of wildly different structures that are not connected evolutionary speaking, but nevertheless employ wildly similar mechanisms, really is strong evidence of design in my view. Sort of like this one:

ATP synthase:


compare to the mechanism of eukaryotic cilia:


[ 29. December 2003, 22:11: Message edited by: Nelson-Alonso ]

posted 30. December 2003 00:32                   

quote:

---

\
Comparisons of the bacteriorhodopsins with other families did not yield significant results. This result suggests that either vast evolutionary distances have caused the TMSs to diverge past the point of recognition, or that GPCRs and bacteriorhodopsins have independent origins as polytopic membrane proteins.

---

quote:

---

ABSTRACT Opsins are a class of retinal-binding, seven transmembrane helix proteins that function as lightresponsive ion pumps or sensory receptors. Previously, genes encoding opsins had been identified in animals and the Archaea but not in fungi or other eukaryotic microorganisms.
Here, we report the identification and mutational analysis of an opsin gene, nop-1, from the eukaryotic filamentous fungus Neurospora crassa. The nop-1 amino acid sequence predicts a protein that shares up to 81.8% amino acid identity with
archaeal opsins in the 22 retinal binding pocket residues, including the conserved lysine residue that forms a Schiff base linkage with retinal. Evolutionary analysis revealed relatedness
not only between NOP-1 and archaeal opsins but also between NOP-1 and several fungal opsin-related proteins that lack the Schiff base lysine residue. The results provide evidence for a eukaryotic opsin family homologous to the archaeal
opsins, providing a plausible link between archaeal and visual opsins. Extensive analysis of Dnop-1 strains did not reveal obvious defects in light-regulated processes under normal laboratory conditions. However, results from Northern analysis support light and conidiation-based regulation of nop-1 gene expression, and NOP-1 protein heterologously expressed in Pichia pastoris is labeled by using all-trans [3H]retinal, suggesting that NOP-1 functions as a rhodopsin in N. crassa photobiology.

---

quote:

---

In conclusion, we have identified statistically significant alignments, across two or more TMSs, indicating probable homology, between the main family of GPCRs and four other families of GPCRs (VIP/secretin receptors, metabotropic glutamate receptors, fungal pheromone receptors, and cAMP receptors). Lack of significant alignments unanimously across all seven TMSs indicates either the divergence of TMSs of a common origin, or the splicing together of genetic material encoding TMSs of independent origins. As the hypothesis of divergence is the simplest, and as there is no evidence pointing to alternative origins of these TMSs, and further, as many of these TMSs produced high-scoring, consistent alignments, we conclude that, in all likelihood, these five families of GPCRs are homologous, across all seven TMSs, and that any dissimilarity between corresponding TMSs is the result of divergence. It should be noted that for the fungal pheromone receptors, we established a link only for the subfamily of mapr. A common origin of the fungal pheromone receptors belonging to the mamr subfamily with other GPCRs remains unproven.

---

posted 30. December 2003 17:12                   

quote:

---

Common design through evolution or through what?

---

quote:

---

After all if natural processes can lead to design, design itself in nature is not very helpful as far as ID is concerned.

---

quote:

---

Comparisons of the bacteriorhodopsins with other families did not yield significant results. This result suggests that either vast evolutionary distances have caused the TMSs to diverge past the point of recognition, or that GPCRs and bacteriorhodopsins have independent origins as polytopic membrane proteins.

---

quote:

---

The nature of protein sequence space may be helpful understanding these issues. Sequence homologies may not be the only relevant measure. We may have to look at structural homologies.

---

quote:

---

Neslon objects to my reference to the thesis which mentions

---

quote:

---

Nelson points out some fascinating homology between ATP and cylia. Fascinating evidence. I wonder what Nelson had in mind here?

---

posted 30. December 2003 23:04                   

quote:

---

Though 7TMRs are currently unknown in eukaryotes other than animals, slime molds, fungi and plants, distantly related proteins, namely the prokaryotic rhodopsins, are encountered in bacteria and archaea [15,16]. The animal and the prokaryotic rhodopsins widely differ from each other in the residues that bind retinal and the actual location of the ligand in the internal pocket. However, structural comparisons between the animal rhodopsins and the prokaryotic proteins reveal that they adopt essentially the same topology and three-dimensional fold [10,17,18]. This suggests that they have most probably descended from a common ancestor despite extensive divergence of their sequence.

---

posted 30. December 2003 23:18                   

quote:

---

Nelson: Denton's paper, discussed here calls into question homology based on structural similarities.

---

quote:

---

On the contrary, and Mike argues that "If there are only a few thousands protein folds, then our degree of confidence about homology is greatly weakened if the main pillar of this inference is based on structural similarity. That is, if we were dealing with a nearly-infinite number of potential protein folds, then the fact that two proteins share folds would be strongly suggestive of common descent. But if the number of structures is quite limited, then an origin through convergence, or common design, is equally plausible. "

---

quote:

---

But if the number of structures is quite limited, then an origin through convergence, or common design, is equally plausible.

---

quote:

---

Of course failing any probability calculations the support for the latter one is hard to establish but I will agree with Mike that FLE in principle does not rule out intelligent design but I do not see much which could rule out ID in this context, or support it for that matter.

---

quote:

---

The fact that in many cases where the same fold is adapted to different functions, no trace of homology can be detected in the amino acid sequences, suggesting multiple separate discoveries of the same basic structure during the course of evolution (Orengo et al., 1994; Brandon & Tooze, 1999), further reinforces the conclusion that the folds are a .finite set of ahistoric physical forms.

---

quote:

---

I would argue that the combination of scale free neutral sequence space may help us understand the appearance of convergence.

---

posted 31. December 2003 01:55                   

quote:

---

Based on these observations, the high degree of divergence between archaeal rhodopsins and NOP-1, and the apparent monophyletic nature of the archaeal group, we conclude that NOP-1 is unlikely to reflect a horizontal gene transfer from a halophilic archaeote.

---

quote:

---

Frida finished her Masters of Science in chemistry at Gothenburg University, 2001, working within the structural biology group of Lennart Sjolin. Shorter afterwards she began her PhD education within the Kinetic Crystallography group at Chalmers. Her project concerns intermediate trapping & time resolved studies on some members of the archael rhodopsin family. It is hoped that the time-resolved work can provide a platform for extending novel ultrafast X-ray scattering methodology into the biological sphere. In parallel Frida is pursuing kinetic crystallography studies on a copper containing nitrite reductase.

---

quote:

---

Summary

We report the first sensory rhodopsin observed in the eubacterial domain, a green light-activated photoreceptor in Anabaena (Nostoc) sp. PCC7120, a freshwater cyanobacterium. The gene encoding the membrane opsin protein of 261 residues (26 kDa) and a smaller gene encoding a soluble protein of 125 residues (14 kDa) are under the same promoter in a single operon. The opsin expressed heterologously in Escherichia coli membranes bound all-trans retinal to form a pink pigment (max 543 nm) with a photochemical reaction cycle of 110 ms half-life (pH 6.8, 18°C). Co-expression with the 14 kDa protein increased the rate of the photocycle, indicating physical interaction with the membrane-embedded rhodopsin, which we confirmed in vitro by affinity enrichment chromatography and Biacore interaction. The pigment lacks the proton donor carboxylate residue in helix C conserved in known retinylidene proton pumps and did not exhibit detectable proton ejection activity. We detected retinal binding to the protein in Anabaena membranes by SDS-PAGE and autofluorography of 3H-labelled all-trans retinal of reduced membranes from the organism. We conclude that Anabaena rhodopsin functions as a photosensory receptor in its natural environment, and suggest that the soluble 14 kDa protein transduces a signal from the receptor. Therefore, unlike the archaeal sensory rhodopsins, which transmit signals by transmembrane helix-helix interactions with membrane-embedded transducers, the Anabaena sensory rhodopsin may signal through a soluble cytoplasmic protein, analogous to higher animal visual pigments.

---

Or see

PNAS Vol. 96, Issue 14, 8034-8039, July 6, 1999
The nop-1 gene of Neurospora crassa encodes a seven transmembrane helix retinal-binding protein homologous to archaeal rhodopsins Jennifer A. Bieszke, Edward L. Braun, Laura E. Bean, Seogchan Kang, Donald O. Natvig, and Katherine A. Borkovich

quote:

---

Fig. 3. Phylogenetic relationship between NOP-1 and related proteins from fungi and archaea. Phylogeny of the opsins was estimated by ML quartet puzzling, NJ of distances with = 2, and MP. Analyses were conducted by using the complete alignment. Estimates of branch lengths were obtained by ML, assuming the PAM model of evolution with empirical amino acid frequencies, and the scale bar indicates 0.1 estimated amino acid substitutions per site (EAASS) by using this model. Support for the monophyly of subgroups of archaeal rhodopsins and all groupings within the fungal proteins is shown, with the quartet puzzling values above branches and the NJ and MP bootstrap proportions below branches. Cases in which alternative groupings were supported by NJ or parsimony are indicated by * (within the fungal proteins, NJ and parsimony bootstrap consensus trees reverse the branching order of C. versicolor Hsp30 and the S. pombe conserved hypothetical protein). Two possible tree roots, and , are indicated by arrows. Cve, Coriolus versicolor; Eni, Emericella nidulans (the sexual form of Aspergillus nidulans); Hsa, Halobacterium salinarum; Hva, Haloarcula vallismortis; Ncr, Neurospora crassa; Nph, Natronomonas pharaonis; Sce, Saccharomyces cerevisiae; Spo, Schizosaccharomyces pombe. International Collaboration (IC) accession numbers: HR Hsa, P16102; cHR-3 Hva, P94853; HR SG1, P25964; HR Nph, P15647; CR-2 Hva, Q53496; BR Hsa, P02945; SR-1 SG1, P19585; AR-2 AUS-2, P29563; SR-I Hsa, P25964; SR-I SG1, P33743; cSR-3 Hva, Q48334; SR-II Hsa, P71411; SR-II Nph, P42196; SR-II Hva, P42197; NOP-1 Ncr, AF135863; ORP Eni, AA787158, AA785169, AA786492; Hsp30 Cve, AB003518; CHP Spo, AL031824; Hsp30p Sce, S31838; YDR033W Sce, S61586; Yro2p Sce, P38079.

---

quote:

---

Archaebacterial rhodopsins, a family of seven helix membrane proteins containing retinal as the chromophore, are receiving more and more interest because they can be considered as archetypes for signal transduction and ion transport. Representatives of this family have now been identified in all three domains of life (Oesterhelt and Stoeckenius, 1971; Bieszke et al., 1999; Beja et al., 2000). Functionally, two different classes can be distinguished. The ion pumps bacteriorhodopsin (BR) and halorhodopsin operate as energy converters, whereas the photoreceptors sensory rhodopsin I (SRI) and II [SRII, also named phoborhodopsin (Tomioka et al., 1986)] provide the initial signal, which enables the cells to seek favourable light conditions (for a recent review on retinal proteins see Spudich et al., 2000). The two photoreceptors are tightly complexed to receptor-specific transducers [Halobacterial transducer I (HtrI) and II (HtrII)]. The incoming light is relayed from the receptor to the cytoplasmic domain of the transducer, which in turn activates the two-component signalling cascade, well known from bacterial chemotaxis as well as from lower eukaryotic species. Both transducers, like the bacterial and archaebacterial chemoreceptors, contain two transmembrane-spanning helices, which are thought to be involved in the signal transfer across the membrane. From structural analysis as well as electron paramagnetic resonance (EPR) measurements, several models have been proposed to explain the signal transfer from the extracellular receptor domain to the cytoplasmic signal domain, which, in general, are based on the relative movement of the two transmembrane helices TM1 and TM2 (Kim, 1994; Chervitz and Falke, 1996; Ottemann et al., 1999).

---

quote:

---

Bacterial Rhodopsin: Evidence for a New Type of Phototrophy in the Sea
Oded Béjà,1 L. Aravind,2 Eugene V. Koonin,2 Marcelino T. Suzuki,1 Andrew Hadd,3 Linh P. Nguyen,3 Stevan B. Jovanovich,3 Christian M. Gates,3 Robert A. Feldman,3 John L. Spudich,4 Elena N. Spudich,4 Edward F. DeLong1*

Extremely halophilic archaea contain retinal-binding integral membrane proteins called bacteriorhodopsins that function as light-driven proton pumps. So far, bacteriorhodopsins capable of generating a chemiosmotic membrane potential in response to light have been demonstrated only in halophilic archaea. We describe here a type of rhodopsin derived from bacteria that was discovered through genomic analyses of naturally occuring marine bacterioplankton. The bacterial rhodopsin was encoded in the genome of an uncultivated -proteobacterium and shared highest amino acid sequence similarity with archaeal rhodopsins. The protein was functionally expressed in Escherichia coli and bound retinal to form an active, light-driven proton pump. The new rhodopsin exhibited a photochemical reaction cycle with intermediates and kinetics characteristic of archaeal proton-pumping rhodopsins. Our results demonstrate that archaeal-like rhodopsins are broadly distributed among different taxa, including members of the domain Bacteria. Our data also indicate that a previously unsuspected mode of bacterially mediated light-driven energy generation may commonly occur in oceanic surface waters worldwide.

---

posted 31. December 2003 17:59                   

quote:

---

Horizontal transfer also makes for some interesting scenarios.

But

The nop-1 gene of Neurospora crassa encodes a seven transmembrane helix retinal-binding protein homologous to archaeal rhodopsins, Proc Natl Acad Sci U S A. 1999 July 6; 96 (14): 8034–8039

---

quote:

---

All-trans-retinal based, light-driven ion pumping and light sensing are no longer an exclusive archaeal enterprise after the exciting discovery of archaeal-type rhodopsins in bacteria and eukarya. Following the discovery of proton-pumping rhodopsins in marine bacteria (proteorhodopsins), an archaetypal system, consisting of a membrane-intrinsic sensory rhodopsin and a soluble interacting transducer, was recently identified in the cyanobacterium Anabaena. The powerful approach that combines genome `digging' and protein expression is rapidly changing our understanding of light responses in lower organisms.

Crossing the borders: archaeal rhodopsins go bacterial
Trends in Microbiology
Volume 11, Issue 9 , September 2003, Pages 405-407

---

quote:

---

The finding of archaeal-like rhodopsins in organisms as diverse as marine proteobacteria and eukarya(6 ) suggests a potential role for lateral gene transfer in their dissemination.

Bacterial Rhodopsin: Evidence for a New Type of Phototrophyin the Sea Science 289(5486):1902-6‚ 2000.

---

quote:

---

Heijne W.H.M., D.W. Smith and M.H. Saier, Jr. (2000). Homologues of archael rhodopsins in plants, animals and fungi

---

quote:

---

Demonstration of a sensory rhodopsin in eubacteria
Kwang-Hwan Jung, Vishwa D. Trivedi and John L. Spudich Volume 47 Issue 6 Page 1513 - March 2003

---

quote:

---

Therefore, unlike the archaeal sensory rhodopsins, which transmit signals by transmembrane helix-helix interactions with membrane-embedded transducers, the Anabaena sensory rhodopsin may signal through a soluble cytoplasmic protein, analogous to higher animal visual pigments.

---

posted 31. December 2003 20:00                   

quote:

---

The nop-1 gene of Neurospora crassa encodes a seven transmembrane helix retinal-binding protein homologous to archaeal rhodopsins, Proc Natl Acad Sci U S A. 1999 July 6; 96 (14): 8034–8039

---

quote:

---

The microbial rhodopsins (MR) are homologous to putative chaperone and retinal-binding proteins of fungi. These proteins comprise a coherent family that we have termed the MR family. We have used modeling techniques to predict the structure of one of the putative yeast chaperone proteins, YRO2, based on homology with bacteriorhodopsins (BR). Availability of the structure allowed depiction of conserved residues that are likely to be of functional significance. The results lead us to predict an extracellular protein folding function and a transmembrane proton transport pathway. We suggest that protein folding is energized by a novel mechanism involving the proton motive force. We further show that MR family proteins are distantly related to a family of fungal, animal and plant proteins that include the human lysosomal cystine transporter (LCT) of man (cystinosin), mutations in which cause cystinosis. Sequence and phylogenetic analyses of both the MR family and the LCT family are reported. Proteins in both families are of the same approximate size, exhibit seven putative transmembrane -helical spanners (TMSs) and show limited sequence similarity. We show that the LCT family arose by an internal gene duplication event and that TMSs 1-3 are homologous to TMSs 5-7. Although the same could not be demonstrated statistically for MR family members, homology with the LCT family suggests (but does not prove) a common evolutionary pathway. Thus, TMSs 1-3 and 5-7 in both LCT and MR family members may share a common origin, accounting for their shared structural features.

---

quote:

---

Ihara et al. [11] con¢rmed the phylogenetic observations of Kuan and Saier [1] and extended the analyses to several newly sequenced proteins, all of which fell into the three groups (BR, HR and SR) observed previously. The family was expanded from 11 members to 25 members, all of which are archaeal retinal containing chromophoric proteins. Ihara et al. [11], however, failed to note that the archaeal retinal-containing chromophoric proteins are, in fact, homologous to yeast and fungal proteins that apparently lack a chromophore and have been shown to be stress-induced [12^15]. These proteins appear to function in cellular responses to acid, organic solvent and heat stress signals [16]. Further, after completion of the analyses reported here, Bieszke et al. [17,18] sequenced a Neurospora crassa protein that encodes a retinal-binding homologue of the archaeal rhodopsins, and Beja et al. [19] identified retinal-containing, light-driven, proton-pumping homologue of the archaeal rhodopsins in a bacterium.
It seems clear that these proteins are widespread in nature.

---

quote:

---

The analyses reported in this communication provide detailed information about conserved residues in putative fungal chaperones and archaeal photoreceptor proteins which, although divergent in function, appear to be conserved in structure. This postulate is based on: (1) common ancestry as revealed by high degrees of sequence similarity (see Fig. 1); (2) common apparent topology as revealed by the hydropathy plots shown in Fig. 2A^D; and (3) the results of average amphipathicity analyses suggesting that these proteins may share biogenic properties as discussed for other classes of integral membrane proteins [41,42]. Of equal significance are the evolutionary implications resulting from our phylogenetic analyses. Thus, homologues of the MR family are found in bacterial, archaeal, and eukaryotic kingdoms. It can be argued that the primordial gene giving rise to all MR family members probably arose before divergence of these three kingdoms from each other.

---

quote:

---

We demonstrate that two rhodopsins, identified from cDNA sequences, function as low- and high-light-intensity phototaxis receptors in the eukaryotic alga Chlamydomonas reinhardtii. Each of the\ receptors consists of an
300-residue seven-transmembrane helix domain with a retinal-binding pocket homologous to that of archaeal
rhodopsins, followed by 400 residues of additional membrane associated portion. The function of the two rhodopsins, Chlamydomonas
sensory rhodopsins A and B (CSRA and CSRB), as phototaxis

---

quote:

---

In the 1970s and early 1980s four rhodopsins were discovered in the cytoplasmic membrane of the archaeon Halobacterium salinarum: the light-driven ion pumps bacteriorhodopsin and halorhodopsin, and the phototaxis receptors sensory rhodopsin I and II. Genome projects on a number of microbes have more recently revealed archaeal rhodopsin homologs in the other two domains of life as well, namely Eubacteria and Eucarya. Organisms containing these homologs live in such diverse environments as soil, freshwater, and ocean waters, and they include a broad
range of microbial life, including proteobacteria, cyanobacteria, fungi, and algae (24). The Chlamydomonas rhodopsins provide
an example of evolution fusing the seven-helix microbial rhodopsin motif with other domains (Fig. 2).

---