Identification and in vivo characterization of PpaA, a regulator of photosystem formation in Rhodobacter sphaeroides

1 Department of Molecular Biology, University of Wyoming, Laramie, WY 82071-3944, USA 2 Department of Microbiology and Parasitology, University of Queensland, Brisbane 4072, Australia 3 Department of Microbiology and Molecular Genetics, University of Texas Medical School, Houston, TX 77030, USA Correspondence Mark Gomelsky gomelsky{at}uwyo.edu A regulatory protein, PpaA, involved in photosystem formation in the anoxygenic phototrophic proteobacterium Rhodobacter sphaeroides has been identified and characterized in vivo . Based on the phenotypes of cells expressing the ppaA gene in extra copy and on the phenotype of the ppaA null mutant, it was concluded that PpaA activates photopigment production and puc operon expression under aerobic conditions. This is in contrast to the function of the PpaA homologue from Rhodobacter capsulatus , AerR, which acts as a repressor under aerobic conditions [Dong, C., Elsen, S., Swem, L. R. & Bauer, C. E. (2002). J Bacteriol 184, 2805–2814]. The expression of the ppaA gene increases several-fold in response to a decrease in oxygen tension, suggesting that the PpaA protein is active under conditions of low or no oxygen. However, no discernible phenotype of a ppaA null mutant was observed under anaerobic conditions tested thus far. The photosystem gene repressor PpsR mediates repression of ppaA gene expression under aerobic conditions. Sequence analysis of PpaA homologues from several anoxygenic phototrophic bacteria revealed a putative corrinoid-binding domain. It is suggested that PpaA binds a corrinoid cofactor and the availability or structure of this cofactor affects PpaA activity. Abbreviations: PS, photosystem; RC, reaction centre; Km, kanamycin; Sm, streptomycin; Sp, spectinomycin; Tc, tetracycline The GenBank accession number for the sequence reported in this paper is L19596 .