Photosynthesis gene regulation in Rubrivivax gelatinosus; transcription factor PpsR is involved in both negative and positive control

Abstract

Induction of the photosystem biosynthesis in anoxygenic photosynthetic bacteria arises in the absence of oxygen. The regulation of this induction occurs at the transcriptional level, with photosynthesis genes expressed preferentially under anaerobic conditions. Here we report analysis of the transcriptional control of three photosynthesis promoters, puc (light-harvesting II apoproteins), crtI (carotenoid biosynthesis) and bchG (bacteriochlorophyll biosynthesis) by the ppsR transcription factor in R. gelatinosus. This was accomplished by analysing the photosytem production in the wild type and in the PpsR null mutant grown under anaerobic and semiaerobic conditions and by assessing the beta galactosidase activity of lacZ transcriptionnal fusion to promoters possessing the putative ppsR binding boxes. It was found that under semi-aerobic conditions, while inactivation of the ppsR gene results in overproduction of carotenoid pigments, the production bacteriochlorophyll was slightely reduced and the production of LHII was completely abolished. These finding suggest that in contrast to what has been found previously for Rhodobacter species, PpsR acts in R. gelatinosus as an aerobe repressor of crtI gene, meanwhile it acts as an aerobe activator for the expression of both pucBA and an unyet identified bch gene. These results were confirmed by the b-galactosidase activities from pucB::lacZ and crtl::lacZ fusions. Inspection of the putative ppsR binding boxes revealed significant differences (sequence and spacing between the boxes) that could explain the different levels of expression of the studied genes and based on these analyses a model addressing the PpsR involvement in positive and negative regulation is proposed.