When available, Sinorhizobium meliloti utilizes C 4 -dicarboxylic acids as preferred carbon sources for growth. It will preferentially utilize these carbon sources for metabolism while suppressing the utilization of alternative carbon sources such as α- and β-galactosides. The phenomenon of using succinate as the sole carbon source in the presence of secondary carbon sources is termed Succinate Mediated Catabolite Repression (SMCR). A genetic screen identified genes which relieved SMCR in cells grown in succinate with lactose, succinate with maltose and succinate with raffinose. With this screen, we identified a two component regulatory system which affected SMCR comprised of genes encoding proteins Sma0113, a histidine kinase with five PAS domains, and Sma0114, a response regulator lacking a DNA-binding domain. In-frame deletion mutants of sma0113 show a relief of catabolite repression, compared to the wild-type. Tetrazolium dye reduction experiments with these same mutants have shown a change in redox activity. We believe that this two component system may use sensory information about redox levels or energy state gathered from the PAS domains of Sma0113 to modulate electron flow and metabolism by catabolite repression.
The wild-type phenotype seen in the sma0114 deletion mutants is not fully understood. The absence of an altered wild-type phenotype is believed to be caused by crosstalk of Sma0113 with another response regulator, with the signal bypassing Sma0114 due to the lack of this protein in the sma0114 deletion strains. The absence of a DNA binding domain on the response regulator suggests that protein-protein interactions are involved in ultimately transmitting the signal from this two component system in the cell.
The role of Sma0114 as well as Sma0113 has been studied through biochemical methods, including initial structural analysis by protein crystallization and protein phosphosphorylation assays with small molecular donors. This was done to determine the roles that the proteins have in the phosphotransfer and how this might propagate information throughout the cell.
