PHYSIOLOGICAL CONTROL AND REGULATION OF THE RHODOBACTER-CAPSULATUS CBB OPERONS

The genes encoding enzymes of the Calvin-Benson-Bassham (CBB) reductiv e pentose phosphate pathway in Rhodobacter capsulatus are organized in at least two operons, each preceded by a separate cbbR gene, encoding potential LysR-type transcriptional activators. As a prelude to studi es of cbb gene regulation in R. capsulatus, the nucleotide sequence of a 4,537-bp region, which included cbbR,, was determined. This region contained the following open reading frames: a partial pgm gene (encod ing phosphoglucomutase) and a complete qor gene (encoding NADPH:quinon e oxidoreductase), followed by cbbR(II), cbbF (encoding fructose 1,6-b isphosphatase), cbbP (encoding phosphoribulokinase), and part of cbbT (encoding transketolase). Physiological control of the CBB pathway and regulation of the R. capsulatus cbb genes were studied by using a com bination of mutant strains and promoter fusion constructs. Characteriz ation of mutant strains revealed that either form I or form II ribulos e 1,5-bisphosphate carboxylase/oxygenase (RubisCO), encoded by the cbb LS and cbbM genes, respectively, could support photoheterotrophic and autotrophic growth. A strain with disruptions in both cbbL and cbbM co uld not grow autotrophically and grew photoheterotrophically only when dimethyl sulfoxide was added to the culture medium. Disruption of cbb P resulted in a strain that did not synthesize form LI RubisCO and had a phenotype similar to that observed in the RubisCO-minus strain, sug gesting that there is only one cbbP gene in R. capsulatus and that thi s gene is cotranscribed with cbbM. Analysis of RubisCO activity and sy nthesis in strains with disruptions in either cbbR(I) or cbbR(II) and beta-galactosidase determinations from wild-type and mutant strains co ntaining cbb(Ip)- and cbb(IIp)-lacZ fusion constructs, indicated that the cbb(I) and cbb(II) operons of R. capsulatus are within separate Cb bR regulons.