Metastable regulation of type 1 piliation in Escherichia coli and isolation and characterization of a phenotypically stable mutant.

Type 1 piliation in Escherichia coli exhibits phase variation due to the inversion of a small, ca. 300-base-pair, element that regulates pilA (fimA), the gene that encodes the structural subunit of pili (Abraham et al., Proc. Natl. Acad. Sci. USA 82:5724-5727, 1985). We have used the inversion as an assay to characterize a stably piliated mutant. The mutant strain did not exhibit the pilA ON and pilA OFF colonial variants characteristic of the wild type; rather, every clone produced a level of pilA expression intermediate between ON and OFF wild-type populations. The mutant phenotype was conferred by a lesion at a previously undescribed locus between hemA and trpA, which we have termed pilG. Examination of the pilA promoter region in four pilG mutant populations indicated that the phenotypic stability conferred by the pilG mutation was not due to an inability to carry out the inversion. Rather, all pilG mutant populations consisted of approximately equal mixtures of ON and OFF individuals. We suggest that pilG mutants may undergo such rapid switching of the pilA promoter that populations exhibit an intermediate level of pilA expression and phenotypic stability.