Regulatory genes in the thermoregulation of Escherichia coli pili gene transcription.

Expression of several different pilus adhesins by Escherichia coli is subject to thermoregulation. The surface-located fimbrial structures are present during growth at 37 degrees C but are not produced by cells grown at lower temperatures, such as 25 degrees C. As a step toward understanding the molecular mechanism, we have studied the role of different cistrons of a cloned pilus adhesin gene cluster (pap) from a uropathogenic E. coli isolate. By promoter cloning, mRNA analysis, and expression of subcloned genes in trans, we have identified the papI gene as the mediator of thermoregulation at the level of pilus adhesin gene transcription. Expression of the major pilus subunit gene (papA) and several other pilus protein cistrons appeared to be dependent on stimulation by the papB and papI gene products. Constructs carrying different pap DNA regions indicated that none of the known Pap proteins acts directly as thermosensor. The chromosomal rpoH gene and RpoH sigma factor did not appear to be required for pap transcription, and the thermoregulation of pilus gene transcription must be different from that of the heat shock regulon. By overexpressing the papI gene product from an expression plasmid in trans, we could circumvent the temperature regulation and turn on production of pilus adhesin at low temperature. Our results suggest that the level of mRNA encoding the PapI activator is limiting at low growth temperatures and that thermoregulation is due to a determinant in the papI-papB intercistronic region.