Abstract
Photosynthesis originated in the domain Bacteria billions of years ago; however, the identity of the last common ancestor to all phototrophic bacteria remains undetermined and speculative. Here I present the evolution of BchF or 3-vinyl-bacteriochlorophyll hydratase, an enzyme exclusively found in bacteria capable of synthetizing bacteriochlorophyll a. I show that BchF exists in two forms originating from an early divergence, one found in the phylum Chlorobi, including its paralogue BchV, and a second form that was ancestral to the enzyme found in the remaining anoxygenic phototrophic bacteria. The phylogeny of BchF is consistent with bacteriochlorophyll a evolving in an ancestral phototrophic bacterium that lived before the radiation event that gave rise to the phylum Chloroflexi, Chlorobi, Acidobacteria, Proteobacteria, and Gemmatimonadetes, but only after the divergence of Type I and Type II reaction centers. Consequently, it is suggested that the lack of phototrophy in many groups of extant bacteria is a derived trait.
Highlights
There are seven major phyla of bacteria known to contain strains with photochemical reaction centers
Form A is found in the phyla Chloroflexi, Acidobacteria, Proteobacteria, and Gemmatimonadetes, and form B is found in the phylum Chlorobi, which included the isoform, BchV
This implies that both forms originated from an early gene duplication event in an ancestral phototroph capable of bacteriochlorophyll a synthesis or alternatively, as a population of ancestral phototrophic bacteria diverged into two new species
Summary
There are seven major phyla of bacteria known to contain strains with photochemical reaction centers These are Cyanobacteria, Proteobacteria, Firmicutes, Chloroflexi, Chlorobi, Acidobacteria, and Gemmatimonadetes [1]. A recent study suggested that a few strains in the phylum Actionabecteria may retain what appears to be a vestigial chlorophyll synthesis pathway [2] From these groups, only Proteobacteria, Chloroflexi, Chlorobi, Acidobacteria, and Gemmatimonadetes are capable of synthetizing bacteriochlorophyll a. The double bond between the C7 and C8 carbons of chlorophyllide a is reduced by chlorophyllide a oxidoreductase, an enzyme made of three subunits, BchX, BchY, and BchZ, or BchXYZ This step is followed by the hydration of the C3-vinyl group of the bacteriochlorophyllide a precursor to form a hydroxyethil group. In the phylum Cyanobacteria, the BchXYZ, BchF, and BchC enzymes are absent; while in Heliobacteria—the phototrophic Firmicutes—BchXYZ catalyzes the direct formation of bacteriochlorophyll g from 8-vinyl-chlorophyllide a, but lack BchF and BchC [5]
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