Early evolution of photosynthesis: clues from nitrogenase and chlorophyll iron proteins.

Chlorophyll (Chl) is often viewed as having preceded bacteriochlorophyll (BChl) as the primary photoreceptor pigment in early photosynthetic systems because synthesis of Chl requires one fewer enzymatic reduction than does synthesis of BChl. We have conducted statistical DNA sequence analyses of the two reductases involved in Chl and BChl synthesis, protochlorophyllide reductase and chlorin reductase. Both are three-subunit enzymes in which each subunit from one reductase shares significant amino acid identity with a subunit of the other, indicating that the two enzymes are derived from a common three-subunit ancestral reductase. The "chlorophyll iron protein" subunits, encoded by the bchL and bchX genes in the purple bacterium Rhodobacter capsulatus, also share amino acid sequence identity with the nitrogenase iron protein, encoded by nifH. When nitrogenase iron proteins are used as outgroups, the chlorophyll iron protein tree is rooted on the chlorine reductase lineage. This rooting suggests that the last common ancestor of all extant photosynthetic eubacteria contained BChl, not Chl, in its reaction center, and implies that Chl-containing reaction centers were a late invention unique to the cyanobacteria/chloroplast lineage.