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 bacteriochlorophyll and carotenoid levels in Rhodobacter sphaeroides revealed two genes: orf-192 and ppsR. The ppsR gene alone is sufficient for photopigment 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 ppsR is homologous to the CrtJ protein of Rhodobacter capsulatus and contains a helix-turn-helix domain that is found in various DNA-binding proteins. Removal of the helix-turn-helix domain renders PpsR nonfunctional. The promoter of ppsR is located within the coding region of the upstream orf-192 gene. When this promoter is replaced by a lacZ promoter, ppsR is expressed in Escherichia coli. An R. sphaeroides DNA fragment carrying crtD', -E, and -F and bchC, -X, -Y, and -Z' exhibited putative promoter activity in E. coli. This putative promoter activity could be suppressed by PpsR in both E. coli and R. sphaeroides. These results suggest that PpsR is a transcriptional repressor. It could potentially act by binding to a putative regulatory palindrome found in the 5' flanking regions of a number of R. sphaeroides and R. capsulatus photosynthesis genes.