Binding of ATP to the RecA protein induces a high affinity DNA binding required for activation of enzyme function. Screens for in vivo recombination and repressor cleavage activities show Gln194 to be intolerant of all substitutions. Analyses of three mutant proteins (Q194N, Q194E, and Q194A) show that although basal enzyme function is maintained, each protein no longer displays an ATP-induced increase in DNA binding affinity. High salt activation of RecA function is also disrupted by these mutations. In contrast, ATP-induced changes in the oligomeric structure of RecA are maintained in the mutant proteins. These results demonstrate that Gln194 is a critical "allosteric switch" for ATP-induced activation of RecA function but is not the exclusive mediator of ATP-induced changes in RecA.