Physiological Control and Regulation of the Rhodobacter capsulatus cbb Operons

Abstract

ABSTRACT The genes encoding enzymes of the Calvin-Benson-Bassham (CBB) reductive 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 studies of cbb gene regulation in R. capsulatus , the nucleotide sequence of a 4,537-bp region, which included cbbR II , was determined. This region contained the following open reading frames: a partial pgm gene (encoding phosphoglucomutase) and a complete qor gene (encoding NADPH:quinone oxidoreductase), followed by cbbR II , cbbF (encoding fructose 1,6-bisphosphatase), 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 combination of mutant strains and promoter fusion constructs. Characterization of mutant strains revealed that either form I or form II ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), encoded by the cbbLS and cbbM genes, respectively, could support photoheterotrophic and autotrophic growth. A strain with disruptions in both cbbL and cbbM could not grow autotrophically and grew photoheterotrophically only when dimethyl sulfoxide was added to the culture medium. Disruption of cbbP resulted in a strain that did not synthesize form II RubisCO and had a phenotype similar to that observed in the RubisCO-minus strain, suggesting that there is only one cbbP gene in R. capsulatus and that this gene is cotranscribed with cbbM . Analysis of RubisCO activity and synthesis in strains with disruptions in either cbbR I or cbbR II , and β-galactosidase determinations from wild-type and mutant strains containing cbb Ip - and cbb IIp - lacZ fusion constructs, indicated that the cbb I and cbb II operons of R. capsulatus are within separate CbbR regulons.