Expression of specific adhesive properties by bacteria in general seems to be regulated to fit the environmental conditions. An example is the transcriptional regulation of digalactoside-specific binding by uropathogenic strains of Escherichia coli. The fimbrial structures (pili) on the bacterial surface carry the adhesin and are present during growth at 37 degrees C but are not produced by cells at lower temperatures, such as 25 degrees C. Thermoregulation of expression is due to temperature-dependent transcription of a regulatory cistron in the pilus-adhesin gene cluster. We have now identified and characterized a new regulatory locus (drdX) and show that a histone-like bacterial protein has an important role in this novel example of thermoregulation of transcription.

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A gene cluster mediating production of pili in uropathogenic Escherichia coli was analysed with respect to regulation of pili synthesis. Two cistrons, papB and papI, were localized upstream of the major pilus subunit gene, papA. The papI‐papB‐papA region was characterized by nucleotide sequencing and by transcriptional analysis. The papA gene was primarily represented by an 800 nucleotide long transcript but was also co‐transcribed with papB as a less abundant 1300 nucleotide long mRNA. Both transcripts presumably terminated at the same site downstream of the papA coding sequence. The weakly expressed papI gene was transcribed in the opposite direction to that of papB and papA. Studies with lacZ operon fusions showed that the papB gene encoded a trans‐active effector required for papA transcription. Similarly, the papI gene stimulated papB transcription in trans. Furthermore, full expression of papA was cis dependent upon the papI‐papB region. Transcription of the papB gene was shown to be dependent upon cAMP and its receptor protein. A binding site for the cAMP‐CRP complex was postulated in the DNA sequence upstream of the papB promoter.

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.

The cAMP receptor protein (CRP) complex (cAMP-CRP) is a global regulator of gene expression. It influences transcription from a number of promoters in Escherichia coli, including two divergently oriented promoters in the pap pili-adhesin gene system. To further defrne the role of cAMP-CRP in pap regulation we monitored protein-DNA interactions in vitro and levels of pap transcription in vivo in wild-type and mutant pap-containing clones. The results showed that activation was mediated by a single cAMP-CRPbinding site centered at nucleotide positions -215.5 and -115.5 relative to the transcriptional start points. A target for thepap-specific regulatory protein PapB was localized adjacent to the cAMP-CRP-binding site. The long-range effects exerted from the protein-binding sites were consistent with the idea that cAMP-CRP caused a change in the local DNA conformation and that a nucleoprotein complex (involving cAMP-CRP and PapB) was formed in the region between the pap promoters. Moreover, transcription became independent of activation of cAMP-CRP and the PapB protein in a mutant lacking the nucleoid-associated protein H-NS. Our findings suggest that the cAMP-CRP complex mediates its positive regulatory function by alleviating transcriptional silencing and, as such, plays a role as antirepressor.The transcriptional activities of genes may be modulated by positive or negative control via the action of DNA-binding proteins (1, 2). In bacteria most activators bind at a distance close enough to the promoter region to allow protein-protein contact with the RNA polymerase, a contact thought important in the activation process (3, 4). The cAMP receptor protein (CRP) of Escherichia coli is involved in activation of many genes. Alternative mechanisms for activation by the cAMP-CRP complex have been suggested. These mechanisms involve either an interaction with RNA polymerase bound to the promoter or structural changes in the DNA from CRP-induced bending, and experimental data supporting both models exist (5-12). Recent studies with different altered promoter structures have shown that transcriptional stimulation by CRP is optimal when the center of its binding site is positioned on one side of the helix, at -41.5 or -61.5 relative to the transcriptional start point, and that stimulation decreases with longer distance (13,14).The cAMP-CRP complex also functions in the regulation ofpap genes that encode digalactoside-binding pili adhesin in uropathogenic E. coli. The pap genes are not expressed in E. coli strains defective in formation of the cAMP-CRP complex, suggesting that the complex has a positive regulatory function in pap gene transcription (15-17). The pap genes are divergently transcribed and are organized into a major operon encoding 10 cistrons (rightward transcription, Fig. 1A) and a monocistronic operon (pap!, leftward transcription; Fig. 1A) (refs. 15, 16, and our unpublished data). The papB gene product also participates in activation ofpap expression. The papB-papI intercistronic region contains sites...