Natural competence in V ibrio cholerae is controlled by a nucleoside scavenging response that requires CytR ‐dependent anti‐activation

Summary Competence for genetic transformation in V ibrio 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 comEA –luciferase fusion identified cytR , encoding the nucleoside scavenging cyt idine r epressor, previously shown in V . cholerae to be a biofilm repressor and positively regulated by TfoX , but not linked to transformation . A Δ 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 E scherichia coli , ‘anti‐activation’ of nucleoside metabolism genes, via protein–protein interactions between critical residues in CytR and CRP ( c AMP r eceptor p rotein), 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, CytR – CRP interactions ‘anti‐activate’ multiple genes, including a possible factor that negatively controls DNA uptake. Thus, a nucleoside scavenging mechanism couples nutrient stress and cell–cell signalling to natural transformation in V . cholerae as described in other bacterial pathogens.