The past decade has witnessed an exiting unveiling of numerous molecular mechanisms that characterize signal transduction by protein-protein interaction. The recent findings encouraged an increasing effort to understand the sequential metabolism of different sugars available as energy sources at the same time. It seems probable that at least three principle mechanisms which act together or separately, mediate carbon catabolite repression (CCR) depending on the system which is under metabolic control: i) by the main signal transducing chain via the ATP-dependent HPr-kinase, HPr(Ser46-P) or alternatively Crh via the central component CcpA and its interaction with cre, ii) by signals sensed from the specific regulators directly or via phosphorylation by HPr, iii) by inducer exclusion based on the concurrence of the enzyme IIA(Glc) domain of the glucose permease, and other PTS-dependent permeases composed only of the B and C domains and lacking the enzyme IIA domain.