Chlorophyll a Biosynthesis Under Anaerobic Environments in the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

The isocyclic ring (E-ring) with 131-oxo group is a common structural feature of all chlorophylls. The E-ring is formed by two structurally unrelated enzymes. One is the oxygen-dependent (AcsF) Mg-protoporphyrin IX monomethyl ester (MPE) cyclase that incorporates molecular oxygen (O2), and the other is the oxygen-independent (BchE) MPE cyclase with water (H2O) as the oxygen donor. Angiosperms and most photosynthetic bacteria employ AcsF and BchE as the sole MPE cyclase, respectively. However, it remains unknown which enzyme operates in cyanobacteria that can thrive in a variety of anaerobic environments on the Earth. There are two acsF candidates, sll1214 (F1) and sll1874 (F2), and three bchE candidates, slr0905 (E1), sll1242 (E2) and slr0309 (E3) in the genome of the cyanobacterium Synechocystis sp. PCC 6803. We isolated five mutants lacking one of these genes. The four mutants other than ▵F1 grew under both aerobic and anaerobic conditions. The ▵F1 mutant failed to grow under aerobic conditions in spite of normal growth under anaerobic conditions. Anomalous accumulation of MPE was detected only in ▵F1 and anaerobically grown ▵F2, but not in any three bchE-mutants. It is suggested that the MPE cyclase in this cyanobacterium is encoded by two acsF-like genes, acsF1 and acsF2 rather than three bchE-like genes, and that AcsF1 is the sole MPE cyclase under aerobic conditions and that AcsF2 operates together with AcsF1 under anaerobic conditions.