A global signal transduction system regulates aerobic and anaerobic CO2 fixation in Rhodobacter sphaeroides

Complementation of a mutant of Rhodobacter sphaeroides defective in photosynthetic CO2 reduction led to the identification of a gene which encodes a protein that is related to a class of sensor kinases involved in bacterial signal transduction. The nucleotide sequence and deduced amino acid sequence led to the finding that the gene which complemented the mutant is the regB (prrB) gene, previously isolated from both R. sphaeroides and Rhodobacter capsulatus and shown to regulate the anaerobic expression of structural genes required for the synthesis of the reaction center and light-harvesting systems of these organisms. The current investigation indicates that in addition to its role in the regulation of photosystem biosynthesis, regB (prrB) of R. sphaeroides is intimately involved in the positive regulation of the cbbI and cbbII Calvin cycle CO2 fixation operons. In addition to regulating the expression of structural genes encoding enzymes of the primary pathway for CO2 fixation in R. sphaeroides, regB was also found to be required for the expression of a gene(s) important for the putative alternative CO2 fixation pathway(s) of this organism. A mutation in regB also blocked expression of structural genes of the cbb regulon in a strain of R. sphaeroides capable of aerobic CO2-dependent growth in the dark. It is thus apparent that regB is part of a two-component system and encodes a sensor kinase involved in the global regulation of both anoxygenic light-dependent- and oxygenic light-independent CO2 fixation as well as anoxygenic photosystem biosynthesis.