CHARACTERIZATION OF AN AEROBIC REPRESSOR THAT COORDINATELY REGULATES BACTERIOCHLOROPHYLL, CAROTENOID, AND LIGHT HARVESTING-II EXPRESSION INRHODOBACTER-CAPSULATUS

For most species of purple photosynthetic bacteria, the presence of mo lecular oxygen represses synthesis of carotenoids and bacteriochloroph yll. In this study we characterize a strain of Rhodobacter capsulatus, DB469, which contains a genomic disruption of an open reading frame i n the photosynthesis gene cluster termed ORF469. Characterization of t he steady-state level of bacteriochlorophyll synthesis demonstrates th at disruption of ORF469 results in a 2.5-fold increase in aerobic synt hesis of bacteriochlorophyll over that observed with the parent strain . Utilizing reporter plasmids that contain transcriptional fusions of lacZ to various carotenoid and bacteriochlorophyll biosynthesis genes, we also demonstrate that disruption of ORF469 leads to an approximate twofold increase in bacteriochlorophyll and carotenoid gene expressio n under anaerobic growth conditions. Similar analysis with reporter pl asmids that contain translational fusions of lacZ to the puf, puh, and puc operons demonstrates that disruption of ORF469 leads to elevated levels of aerobic transcription of light harvesting-II genes (puc), wi thout affecting light harvesting-I or reaction center gene expression (puf and puh, respectively). Gel mobility analysis demonstrates that D B469 cells lack a DNA-binding protein that interacts with a palindromi c sequence in the bchC promoter region. The results of this study indi cate that ORF469 codes for a DNA-binding protein that acts as an aerob ic repressor of promoters for bacteriochlorophyll, carotenoid, and lig ht harvesting-ii gene expression.