THE MOLECULAR REGULATION OF THE REDUCTIVE PENTOSE-PHOSPHATE PATHWAY IN PROTEOBACTERIA AND CYANOBACTERIA

In phototrophic and chemoautotrophic proteobacteria: genes encoding en zymes of the Calvin-Benson-Bassham pathway of CO2 fixation are often f ound in clusters that are transcribed from a single promoter under con trol of the LysR-type transcriptional activator, CbbR. Mutations affec ting CbbR prevent induction of cbb genes. Gel-retardation assays have demonstrated CbbR binding to putative regulatory regions of cbb operon s, and in two cases, footprinting experiments have delimited the nucle otide sequence protected by CbbR. Fusion of cbh control sequences to r eporter genes has allowed the regions required for promoter activity t o be defined, and recent experiments indicate that the cbb regulon in Rhodobacter is controlled by a global two-component signal transductio n system that also regulates other metabolic processes in this organis m. Different ways of regulating CBB cycle enzymes that also have roles in heterotrophic metabolism have recently been discovered. In cyanoba cteria, the genes of the CBB pathway are organized and regulated diffe rently, and these oxygen-evolving phototrophic bacteria have evolved d ifferent strategies to control the assimilation of CO2.