FIG. 4.

Model depicting carbohydrate catabolite repression of the levD, fruA, and manL genes in S. gordonii. Binding of the transcriptional activator LevR to the levD (or fruA) promoter is negatively impacted by HPr(Ser-P) and underphosphorylated ManL. Conversely, negative regulation of manL by CcpA could be augmented by ManL and HPr(Ser-P). Similar to the findings in S. mutans, ManL in S. gordonii is capable of repressing the expression of levD and fruA, regardless of whether CcpA is present. Whereas CcpA-independent CCR in S. mutans is strongly elicited by glucose or mannose and but not particularly effective when fructose or galactose is the growth carbohydrate, ManL-dependent CCR in S. gordonii does not show such sugar specificity.