Regulation of aerobic succinate transporter dctA of E. coli by cAMP-CRP, DcuS-DcuR and EIIAGlc: Succinate as a carbon substrate and signaling molecule.

Introduction C4-dicarboxylates (C4-DC) emerged as significant growth substrates and signaling molecules for Enterobacteriaceae during colonization of mammalian hosts. The study explored the transcriptional control of the main aerobic C4-DC transporter, dctA, under different carbohydrate conditions. Mutants related to carbon catabolite repression (CCR) and C4-DC regulation (DcuS-DcuR) were examined for better understanding their regulatory integration in aerobic C4-DC metabolism. Additionally, the interaction between the aerobic C4-DC transporter DctA and EIIAGlc from the glucose-specific phosphotransferase system was investigated. Methods The expression of dctA was characterized in the presence of various carbohydrates and regulatory mutants. This was accomplished by fusing the dctA promoter (PdctA) to the lacZ reporter gene. The interaction between DctA and EIIAGlc was examined in vivo using a bacterial two-hybrid system. Results The dctA promoter region contains a class I cAMP-CRP-binding site at position -81.5 and a DcuR-binding site at position -105.5. DcuR, the response regulator of DcuS-DcuR , and cAMP-CRP stimulate dctA expression. The expression of dctA is subject to the influence of various carbohydrates via cAMP-CRP, which modulate cAMP levels. Here we show that EIIAGlc strongly interacts with DctA, potentially resulting in the exclusion of C4-DCs when preferred carbon substrates, such as sugars, are present. In contrast to the classical inducer exclusion known for lactose permease LacY, inhibition of C4-DC uptake into the cytoplasm affects only its role as a substrate, but not as an inducer since DcuS detects C4-DCs in the periplasmic space ('substrate exclusion'). The work shows an interplay between cAMP-CRP and the DcuS-DcuR regulatory system for the regulation of dctA at both transcriptional and post-translational levels. Conclusion The study highlights a hierarchical interplay between global (cAMP-CRP) and specific (DcuS-DcuR) regulation of dctA at the transcriptional and post-translational levels. The integration of global and specific transcriptional regulation, along with the influence of EIIAGlc on DctA, fine-tune C4-DC catabolism in response to the availability of preferred carbon sources. It attributes DctA a central role in the control of aerobic C4-DC catabolism and suggests a new role to EIIAGlc on transporters (control of substrate uptake by substrate exclusion).