Abstract
The transcriptional response of the model cyanobacterium Synechocystis sp. PCC 6803 to low-oxygen conditions includes the up-regulation of several clusters of genes related to photosynthesis and electron transport. One of these clusters includes genes for isoforms of the Rieske protein of the cytochrome b6f complex (encoded by petC2) and the D1 subunit of Photosystem II (PS II; encoded by psbA1). Low-oxygen induced psbA genes are found in the genomes of a number of cyanobacteria; however, their function is not understood. The aim of this project was to determine the roles of psbA1 and the genes of the low-oxygen cluster (LOC) in Synechocystis sp. PCC 6803 under low-oxygen conditions. The LOC genes were co-transcribed in response to fluctuating oxygen levels. None of the genes in the LOC were essential for growth under low-oxygen conditions; however, a psbA1 knockout strain displayed increased respiratory activity and perturbed variable chlorophyll a fluorescence induction. A petC2 knockout strain also displayed altered physiology, suggestive of modulation of quinone exchange by the cytochrome b6f complex. A series of psbA mutant strains were constructed, in which the D1ʹ-encoding psbA1 or the D1-encoding psbA2 were expressed from either the low-oxygen responsive psbA1 promoter or the constitutive psbA2 promoter. These strains were used to characterise the D1ʹ- or D1-containing PS II reaction centres in both low-oxygen and aerobic conditions, and revealed differences between the chlorophyll a fluorescence characteristics and electron transport capabilities of each type. D1ʹ-containing PS II reaction centre complexes were assembled to lower levels than D1-containing centres, probably due to decreased accumulation of the D1ʹ protein. Centres with D1ʹ were more sensitive to light-induced damage than those with D1, leading to an increased susceptibility to photoinhibition of D1ʹ-containing centres. In addition, a serendipitously discovered point mutation in the chlH gene, encoding part of the Mg-chelatase enzyme of chlorophyll a biosynthesis, was characterised. Introduction of the Gly195Glu substitution resulted in a ~40% decrease in the accumulation of chlorophyll a and altered photosystem stoichiometry. The reduced chlorophyll levels in the G195E background may serve as a tool to investigate the biogenesis and repair of the photosynthetic apparatus.