Abstract
Chlorophyll has the fifth ring (E-ring) as a common structural feature. E-ring is formed by two structurally unrelated enzymes, aerobic (AcsF) and anaerobic (BchE) Mg-protoporphyrin IX monomethylester (MPE) cyclases, which involve incorporation of an oxygen atom from molecular oxygen and water, respectively. It has remained unknown which enzyme operates in cyanobacteria. There are two acsF-like genes and three bchE-like genes in the genome of Synechocystis sp. PCC 6803. To identify the gene encoding MPE cyclase, five mutants lacking one of these genes were isolated. Only two mutants lacking acsF-like genes showed growth retardation with anomalous accumulation of MPE under aerobic or anaerobic conditions. Together with the result of RT-PCR, it is suggested that two acsF-like genes not any bchE-like genes encode MPE cyclases, and the two acsF-like genes are differentially regulated by oxygen levels of the environments in Synechocystis sp. PCC 6803.
