What is the essence of a photosynthetic organism?
Organisms capable of photosynthesis are the most numerous class on Earth and arguably the most important, since virtually all life depends on their ability to convert the energy from sunlight into a form accessible to other organisms. What is required for the photosynthetic life? Of course there's the obvious: protein that collect light, proteins that transduce light energy to metabolic energy, enzymes related to carbon metabolism... but what about the not so obvious?

One way to attack this question is to tabulate all proteins that are found in common amongst photosynthetic organisms, regardless of other variations in their lifestyles. It is not as easy as it might sound to compare the proteins of two organisms and decide which are common to both and which are unique -- that's the problem of determining orthologs.

Suppose that we overcome this obstacle in a reasonable way. Take a look at the partial list of common protein amongst six diverse photosynthetic bacteria that results from the comparison of their protein.

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