An operon mediating biogenesis of digalactoside-binding pilus-adhesin of serotype F13 in uropathogenic Escherichia coli includes the regulatory gene papB. The papB gene product was found to act as transcriptional activator of an operon which includes the papB gene and several pap cistrons encoding the proteins of the pilus polymer. Studies of how pap gene expression was affected by increasing amounts of PapB protein in the cells showed that high levels did not stimulate transcription but caused repression. Results from in vitro studies demonstrated that the PapB protein was a sequence-specific DNA-binding protein. Binding studies using gel mobility shift assays and DNase I protection (footprinting) showed that PapB protein binds to three separate sites. A sequence greater than 200 bp upstream of the promoter, and directly adjacent to a binding site for the cAMP receptor protein-cAMP complex, appeared as a preferential PapB binding site. A second site was localized to sequences overlapping the -10 region of the promoter and a third binding site was found within the coding sequence of the papB gene itself. The data suggest that the PapB protein has a dual function as activator/repressor of pilus-adhesin transcription and that its autoregulatory mode of action involves differential binding to separate sites.