SEQUENCING, CHROMOSOMAL INACTIVATION, AND FUNCTIONAL EXPRESSION IN ESCHERICHIA-COLI OF PPSR, A GENE WHICH REPRESSES CAROTENOID AND BACTERIOCHLOROPHYLL SYNTHESIS IN RHODOBACTER-SPHAEROIDES

Sequencing of a DNA fragment that causes trans suppression of bacterio chlorophyll and carotenoid levels in Rhodobacter sphaeroides revealed two genes: orf-192 and ppsR. The ppsR gene alone is sufficient for pho topigment suppression. Inactivation of the R. sphaeroides chromosomal copy of ppsR results in overproduction of both bacteriochlorophyll and carotenoid pigments. The deduced 464-amino-acid protein product of pp sR is homologous to the CrtJ protein of Rhodobacter capsulatus and con tains a helix-turn-helix domain that is found in various DNA-binding p roteins. Removal of the helix-turn-helix domain renders PpsR nonfuncti onal. The promoter of ppsR is located within the coding region of the upstream orf-192 gene. When this promoter is replaced by a lacZ promot er, ppsR is expressed in Escherichia coli. An R. sphaeroides DNA fragm ent carrying crtD', -E, and -F and bchC, -X, -Y, and -Z' exhibited put ative promoter activity in E. coli. This putative promoter activity co uld be suppressed by PpsR in both E. coli and R. sphaeroides. These re sults suggest that PpsR is a transcriptional repressor. It could poten tially act by binding to a putative regulatory palindrome found in the 5' flanking regions of a number of R. sphaeroides and R. capsulatus p hotosynthesis genes.