The parental structure of bacteriochlorophyll a, bacteriochlorin, is formed by a sequential operation of two nitrogenase-like enzymes, dark-operative protochlorophyllide oxidoreductase (DPOR) and chlorophyllide a oxidoreductase (COR). Both DPOR and COR consist of two components, Fe protein and MoFe protein cognates. Here we determined kinetic parameters of COR and established the reconstitution system for the formation of bacteriochlorin (3-vinyl bacteriochlorophyllide a) from porphyrin (protochlorophyllide) with purified components of DPOR and COR from Rhodobacter capsulatus. This reconstitution system confirmed the recent finding that COR catalyzes 8-vinyl reduction of 8-vinyl chlorophyllide a in addition to the known activity of C7C8 double bond reduction, and provides a promising model to investigate how two nitrogenase-like enzymes are coordinated in bacteriochlorophyll biosynthesis.

The fifth ring (E-ring) of chlorophyll (Chl) a is produced by Mg-protoporphyrin IX monomethyl ester (MPE) cyclase. There are two evolutionarily unrelated MPE cyclases: oxygen-independent (BchE) and oxygen-dependent (ChlA/AcsF) MPE cyclases. Although ChlA is the sole MPE cyclase in Synechocystis PCC 6803, it is yet unclear whether BchE exists in cyanobacteria. A BLAST search suggests that only few cyanobacteria possess bchE. Here, we report that two bchE candidate genes from Cyanothece strains PCC 7425 and PCC 7822 restore the photosynthetic growth and bacteriochlorophyll production in a bchE-lacking mutant of Rhodobacter capsulatus. We termed these cyanobacterial bchE orthologs "chlE."