Bacteriorhodopsin gene

Sequencing has shown that the repeated sequences are insertion sequences similar to those widely distributed in the bactena and in eucarya. The first of these characterized was ISHl, an insertion in the bacteriorhodopsin gene of Halobacterium halobium S9. The... 

Dunn R, McCoy J, Simsek M, et al. The bacteriorhodopsin gene. Proc Natl Acad Sci USA 1981 78 6744-6748. 

Many of the early genetic studies were done on H. halobium and its related strains. The popularity of these strains stemmed from the fact that several interesting spontaneous mutations could be readily detected. Among the most characterized mutations are those that affect the production of the protein part of the purple membrane, the heavily studied light-driven proton pump bacteriorhodopsin. Spontaneous mutations occur at a frequency of lO 4. Analysis of these mutations showed that in almost every case a foreign DNA sequence was introduced into the bacterioopsin (bop) structural gene or into sequences surrounding it. 

The second line of evidence relates to more recent metabolic innovations such as photosynthesis. LUCA, it seems, could not photosynthesize. No form of photosynthesis based on chlorophyll is found in any archaea. A completely different form of photosynthesis, based on a pigment called bacteriorhodopsin, similar to the photoreceptor pigments in our eyes, is practised by the so-called halobacteria, archaea that live in high-salt conditions. This mode of photosynthesis is not found in any bacteria. These disparate forms of photosynthesis presumably evolved indepen-dently in bacterial and archaeal lineages some time after the age of LUCA, and subsequently remained tied to their respective domains. If a metabolic innovation as important as photosynthesis did not cross from one domain to another, there is no reason to think that other forms of respiration would have done so. We should certainly be wary of postulating that respiratory genes crossed domains unless we have evidence that they did so and the evidence from evolutionary trees suggests that they did not. 

Even though rhodopsin and bacteriorhodopsin appear to differ fundamentally in their function, mode of action, structure, and photochemistry, it has been suggested (Stoeckenius et ai, 1979) that the two proteins did not evolve independently. One possibility is that the halobacteria may have acquired bacteriorhodopsin by gene transfer from a eukaryote (Stoeckenius et al., 1979). However, a preliminary report by Hargrave et al. (1983) claims that no statistically significant sequence homology can be found. Thus, the resemblance between these proteins appears to be only superficial. 

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