Multiple regulators and their interactions in vivo and in vitro with the cbb regulons of Rhodobacter capsulatus

Abstract

The cbbI and cbbII operons encode structural genes which are important for carbon dioxide fixation via the Calvin-Benson-Bassham reductive pentose phosphate pathway in Rhodobacter capsulatus. Each operon is regulated by cognate LysR-type transcriptional activators, CbbRI and CbbRII, with the product of the cbbRI gene, CbbRI, able to control its own transcription under some growth conditions. Furthermore, CbbRI may at least partially regulate the cbbII operon, with significant, yet regulated transcription of the cbbII operon occurring in the absence of any CbbR. These results suggested the importance of additional regulators. Thus, in addition to the rather specific control exerted by CbbR, a more globally significant regulatory system, the RegA-RegB (PrrA-PrrB) two-component system, was found to contribute to transcriptional regulation of each cbb operon. The regA and regB mutant strains were found to contain constitutive levels of form I and form II RubisCO, the major proteins encoded by the cbbI and cbbII operons, respectively. In addition, DNaseI footprint analyses indicated that RegA∗, a constitutively active mutant form of RegA, binds specifically to cbbI and cbbII promoter-operator regions. CbbRI, CbbRII, and RegA binding loci were localized relative to transcription start sites, leading to a coherent picture of how each of these regulators interacts with specific promoter-operator sequences of the cbb operons.