The molecular mechanism for carbon catabolite repression of the chitin response in Vibrio cholerae

Vibrio cholerae is a facultative pathogen that primarily occupies marine environments. In this niche, V. cholerae commonly interacts with the chitinous shells of crustacean zooplankton. As a chitinolytic microbe, V. cholerae degrades insoluble chitin into soluble oligosaccharides. Chitin oligosaccharides serve as both a nutrient source and an environmental cue that induces a strong transcriptional response in V. cholerae. Namely, these oligosaccharides induce the chitin sensor, ChiS, to activate the genes required for chitin utilization and horizontal gene transfer by natural transformation. Thus, interactions with chitin impact the survival of V. cholerae in marine environments. Chitin is a complex carbon source for V. cholerae to degrade and consume, and the presence of more energetically favorable carbon sources can inhibit chitin utilization. This phenomenon, known as carbon catabolite repression (CCR), is mediated by the glucose-specific Enzyme IIA (EIIAGlc) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS). In the presence of glucose, EIIAGlc becomes dephosphorylated, which inhibits ChiS transcriptional activity by an unknown mechanism. Here, we show that dephosphorylated EIIAGlc interacts with ChiS. We also isolate ChiS suppressor mutants that evade EIIAGlc-dependent repression and demonstrate that these alleles no longer interact with EIIAGlc. These findings suggest that EIIAGlc must interact with ChiS to exert its repressive effect. Importantly, the ChiS suppressor mutations we isolated also relieve repression of chitin utilization and natural transformation by EIIAGlc, suggesting that CCR of these behaviors is primarily regulated through ChiS. Together, our results reveal how nutrient conditions impact the fitness of an important human pathogen in its environmental reservoir.Author summary: Vibrio cholerae is a facultative human pathogen that most commonly inhabits aquatic environments and can cause the disease cholera when ingested in the form of contaminated food or drinking water. The survival of this pathogen in the marine environment is dependent upon its ability to colonize chitin, an insoluble polysaccharide that is the main constituent of the shells of crustacean zooplankton. V. cholerae degrades chitin into soluble oligomers, which can be taken up by this microbe and used as a nutrient. Importantly, chitin oligomers also induce the activity of a chitin-sensing protein, ChiS, to activate the expression of genes required for both (1) eating chitin and (2) undergoing horizontal gene transfer by natural transformation. Previous work demonstrated that more energetically favorable carbon sources, like glucose, can inhibit chitin-induced behaviors. However, the mechanism underlying this response remained unclear. Here, we show how a dephosphorylated intermediate in the glucose uptake pathway–EIIAGlc–interacts with ChiS to repress its activity. Furthermore, we demonstrate that this repression of ChiS by EIIAGlc is responsible for inhibiting both growth on chitin and natural transformation. Together, our results reveal how nutrient conditions impact the fitness of a major human pathogen in its environmental reservoir.