Natural competence in Vibrio cholerae is controlled by a nucleoside scavenging response that requires CytR-dependent anti-activation

Competence for genetic transformation in Vibrio cholerae is triggered by chitin-induced transcription factor TfoX and quorum sensing (QS) regulator HapR. Transformation requires expression of ComEA, described as a DNA receptor in other competent bacteria. A screen for mutants that poorly expressed a comEAluciferase fusion identified cytR, encoding the nucleoside scavenging cytidine repressor, previously shown in V.?cholerae to be a biofilm repressor and positively regulated by TfoX, but not linked to transformation. A ?cytR mutant was non-transformable and defective in expression of comEA and additional TfoX-induced genes, including pilA (transformation pseudopilus) and chiA-1 (chitinase). In Escherichia coli, anti-activation of nucleoside metabolism genes, via proteinprotein interactions between critical residues in CytR and CRP (cAMP receptor protein), is disrupted by exogenous cytidine. Amino acid substitutions of the corresponding V.?cholerae CytR residues impaired expression of comEA, pilA and chiA-1, and halted DNA uptake; while exogenous cytidine hampered comEA expression levels and prevented transformation. Our results support a speculative model that when V.?cholerae reaches high density on chitin, CytRCRP interactions anti-activate multiple genes, including a possible factor that negatively controls DNA uptake. Thus, a nucleoside scavenging mechanism couples nutrient stress and cellcell signalling to natural transformation in V.?cholerae as described in other bacterial pathogens.