CbbR and RegA regulate cbb operon transcription in Ralstonia eutropha H16

The biotechnologically important Gram-negative beta-proteobacterium Ralstonia eutropha H16 is able to grow lithoautotrophically by utilizing CO2 and H-2 as sole carbon and energy sources, respectively. CO2 is fixed by the CBB cycle, which is encoded in duplicate on the genome of R. eutropha H16. The transcription of both cbb operons is controlled by the transcription regulator CbbR dependent on intracellular PEP levels as a response to the carbon-state of the cell. As demonstrated in this study transcription control of both cbb operons appears to be more complex and additionally involves, next to CbbR, the transcription regulator RegA as part of the global transcription regulation system RegA/RegB. The identification of a highly conserved RegA/RegB homologue in R. eutropha H16 and experimental evidence gathered in this study reveal that RegA plays a crucial role in the transcription control of both cbb promoters. RegA is able to induce cbb promoter activity and controls transcription in combination with CbbR dependent on cellular PEP concentrations. These results clearly demonstrate that RegA plays an important role in cbb operon transcription regulation and may also be relevant for the control of other energy-utilizing and energy-generating pathways of R. eutropha H16. In addition to promoting a more complete understanding of the CO2 fixation mechanism of R. eutropha H16 these findings also provide crucial insights for the utilization of this bacterium in biotechnological applications with respect to CO2 fixation.