Rhodobacter capsulatus is a nonsulfur purple photosynthetic bacterium that possesses two cbb operons, cbb I and cbb II , encoding enzymes involved in the Calvin-Benson-Bassham (CBB) reductive pentose phosphate pathway of carbon dioxide fixation (24, 25) (Fig. 1). The cbb I operon of R. capsulatus contains cbbL and cbbS, encoding the large and small subunits of form I ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RubisCO), respectively, as well as two genes, cbbQ and cbbO, of unknown function. Aside from cbbM, which encodes form II RubisCO, the cbb II operon also contains cbbF, encoding fructose-1,6/sedoheptulose-1,7-bisphosphatase; cbbP, encoding phosphoribulokinase; cbbT, encoding transketolase; cbbG, encoding glyceraldehyde 3-phosphate dehydrogenase; cbbA, encoding fructose-1,6-bisphosphate aldolase; cbbM, encoding the large subunit of form II RubisCO; cbbE, encoding ribulose-5-phosphate-3-epimerase; cbbZ, encoding 2-phosphoglycolate phosphatase; and cbbY, encoding an unknown function, as well as three unidentified open reading frames.Divergently transcribed from the cbb I and cbb II operons are cbbR I and cbbR I , respectively, which encode regulators that positively affect transcription of their cognate operons. R. capsulatus cbb I and cbb II are regulated independently, and their expression levels have been shown to vary depending on the growth conditions (13,14,24,25,29,43). The level of cbb I and cbb II expression is maximal under photoautotrophic conditions, when the CBB pathway is used to synthesize organic carbon from CO 2 to support growth and maintain the redox balance of the cell (18, 46). During photoheterotrophic growth (i.e., anaerobic growth conditions in the presence of an organic carbon source), cbb II expression is reduced, while cbb I is not expressed. Under aerobic chemoheterotrophic conditions, when CO 2 fixation is not needed, the level of cbb expression is lowest. Regulatory cross talk between the two operons also occurs, since inactivation of either of the two cbb operons in R. capsulatus leads to a compensatory increase in the expression of the remaining operon (25).While the RegB-RegA two-component regulatory system is also involved in derepressing both cbb I and cbb II operon expression under photoautotrophic growth conditions (43), CbbR I and CbbR II for the most part specifically regulate their cognate operons. However, there is some indication that, in the absence of CbbR II , CbbR I may cross regulate cbb II operon expression under photoheterotrophic conditions (43). In addition, partial expression of the cbb II operon occurs under photoautotrophic conditions in the complete absence of CbbR I