NUCLEOTIDE-SEQUENCE AND TRANSCRIPTIONAL ANALYSIS OF THE FLANKING REGION OF THE GENE (SPB) FOR THE TRANS-ACTING FACTOR THAT CONTROLS LIGHT-MEDIATED EXPRESSION OF THE PUF OPERON IN RHODOBACTER-SPHAEROIDES

We recently reported the existence of a trans-acting factor (SPB) in R hodobacter sphaeroides that repressed the expression of the puf operon during illumination. SPB was somewhat homologous to HvrA of Rhodobact er capsulatus, but these proteins appear to have functionally differen t properties. We now report an analysis of the flanking region of spb in the genome of R. sphaeroides, and we show that spb is a positional counterpart of hvrA of R. capsulatus. The region directly downstream o f spb was found to contain three genes, two of which were highly homol ogous to orf5 and ahcY in R. capsulatus. However, a gene corresponding to hvrB, which controls the expression of orf5 and ahcY in R. capsula tus, was absent in R. sphaeroides. The level of the transcript of ahcY did not change in cells grown under photosynthetic and by respiratory conditions. By contrast, orf5 was transcribed at a higher rate in pho tosynthetically grown cells under high-intensity light than under low- intensity light, indicating features of transcription different from t hose in R. capsulatus. A third gene, orf318, which was absent in the c orresponding region of R. capsulatus, encoded an amino acid sequence t hat was significantly homologous to the consensus sequence of RfaI and RfaJ of E. coli, which are glycosyl transferases involved in the synt hesis of lipopolysaccharide. orf318 was transcribed in the opposite di rection to ahcY, and at only a low level, under all conditions tested.