Cross communication between components of carbon catabolite repression of Lactobacillus casei and Bacillus megaterium

In low-GC Gram-positive bacteria, carbon catabolite repression (CCR) is exerted by transcriptional regulation through a protein complex consisting of catabolite control protein CcpA and serine phosphorylated phosphocarrier protein HPr (HPr-ser-P). We investigated the interaction between these components of Lactobacillus casei and Bacillus megaterium. CcpA of L. casei could not complement a B. megaterium ccpA mutant strain, whereas it was found to be functional in Bacillus subtilis. To explore the nature of the non-complementing phenotype, we overproduced and purified CcpA and HPr of L. casei for in vitro analyses. Electrophoretic mobility shift assays revealed a failure in CCR signal transduction at the level of protein-protein interaction between L. casei CcpA and B. megateriumHPr-ser-P, while binding of CcpA to the B. megaterium target site was intact. We established a method based on surface plasmon resonance that allowed a quantitative analysis of CcpA/HPr-ser-P interactions. Calculation of the apparent dissociation constants revealed that the interaction of L. casei CcpA with B. megaterium HPr-ser-P was fivefold weaker than with its own HPr-ser-P suggesting that the reduced affinity was responsible for the non-complementing phenotype.