In vitro activation and repression of photosynthesis gene transcription inRhodobacter capsulatus

It has been known for over half a century that anoxygenic photosynthetic ba cteria maximally synthesize their photosystems in the absence of oxygen. Du ring the last decade, it has become clear that this regulation is largely a t the transcriptional level, with photosynthesis genes expressed only under anaerobic conditions. We describe here in vitro reconstitution of activati on and repression of three photosynthesis promoters, bch (bacteriochlorophy ll biosynthesis), puc (light-harvesting II apoproteins) and puf (reaction c entre and light-harvesting I apoproteins) using purified transcription fact ors and RNA polymerase from Rhodobacter capsulatus. Previous genetic result s have indicated that each of these three promoters is differentially regul ated by three key regulators: CrtJ acting as a repressor of bch and puc and the two-component regulators RegA/RegB, which are activators of puc and pu f. These regulators are distinct from those that mediate oxygen control in enteric bacteria. Our in vitro studies show that these purified regulators directly control the expression of the housekeeping RNA polymerase at these promoters. High-level basal expression of the bch promoter is shown to be repressed by CrtJ. The puc promoter is activated by the RegB-phosphorylated RegA protein and additionally repressed by CrtJ. At the puc promoter, CrtJ effectively competes for promoter binding with RegA, while at the bch prom oter, repression appears to be by competition for the RNA polymerase bindin g site. In contrast to what has been suggested previously, the RegA-activat ed puf promoter is demonstrated as being recognized by the housekeeping RNA polymerase. We also discuss evidence that RegA similar to P activation of the puc and puf promoters involves recruitment of RNA polymerase by differe nt modes of protein-protein interaction.