Citrate Utilization by Corynebacterium glutamicum Is Controlled by the CitAB Two-Component System through Positive Regulation of the Citrate Transport Genes citH and tctCBA

In this work, the molecular basis of aerobic citrate utilization by the gram-positive bacterium Corynebacterium glutamicum was studied. Genome analysis revealed the presence of two putative citrate transport systems. The permease encoded by citH belongs to the citrate-Mg2+: H+/citrate-Ca2+:H+ symporter family, whereas the permease encoded by the tctCBA operon is a member of the tripartite tricarboxylate transporter family. The expression of citH or tctCBA in Escherichia coli enabled this species to utilize citrate aerobically, indicating that both CitH and TctABC are functional citrate transporters. Growth tests with the recombinant E. coli strains indicated that CitH is active with Ca2+ or Sr2+ but not with Mg2+ and that TctABC is active with Ca2+ or Mg2+ but not with Sr2+. We could subsequently show that, with 50 mM citrate as the sole carbon and energy source, the C. glutamicum wild type grew best when the minimal medium was supplemented with CaCl2 but that MgCl2 and SrCl2 also supported growth. Each of the two transporters alone was sufficient for growth on citrate. The expression of citH and tctCBA was activated by citrate in the growth medium, independent of the presence or absence of glucose. This activation was dependent on the two-component signal transduction system CitAB, composed of the sensor kinase CitA and the response regulator CitB. CitAB belongs to the CitAB/DcuSR family of two-component systems, whose members control the expression of genes that are involved in the transport and catabolism of tricarboxylates or dicarboxylates. C. glutamicum CitAB is the first member of this family studied in Actinobacteria.