In Escherichia coli, transport and catabolism of nucleosides require expression of the genes composing the CytR regulon. Transcription initiation of cistrons in this gene family is activated by cyclic AMP-catabolite activator protein (cAMP-CAP), repressed by the CytR protein, and induced by cytidine. A random proofreading mutagenesis procedure and a genetic screen using udp-lac fusions have allowed the identification of distinct regions of the 341-amino-acid CytR polypeptide that are critical for repression of gene expression and response to induction. Determination of the ability of various CytR mutants to control gene expression in vivo indicated that the intrinsic affinity of the CytR protein for operator DNA is gene specific and that efficient repression of transcription by wild-type CytR is dependent on the interaction of CytR with cAMP-CAP. CytR mutants that were cytidine induction defective (CID) were characterized; these mutant proteins had only Asp-281 replaced. Data obtained with cytR delta M149, a dominant negative allele, indicated that the native CytR repressor is an oligomeric protein. Representative cytR mutations were combined with cytR delta M149, and the resulting hybrid repressors were tested for transdominance in a CytR+ E. coli strain. Amino acid substitutions A209E and C289Y suppressed the transdominance of CytR delta M149, suggesting that these replacements alter the normal protein contacts involved in repressor subunit-subunit association. In contrast, amino acid substitutions located in the N-terminal portion of the CytR protein had no effect on the transdominance of CytR delta M149. The results from this study suggest that the CytR repressor is an oligomeric, allosteric protein in which conformational changes are required for repression and derepression.