Roles offimB andfimE in phase variable expression of type 1 fimbriae.

Abstract

The roles of fimB and fimE in the phase-variable expression of type 1 fimbriae in Escherichia coli were elucidated using molecular genetic techniques. Expression of type 1 fimbriae in Escherichia coli was found to involve two distinct modes of phase variation. The first of these is the well characterized on-off phase variation associated with recombination of the fim invertible element. Evidence from both recombinant plasmid- and chromosomal-based assays are consistent with the following model of the roles of fimB and fimE in recombination; fimB promotes recombination in both directions, whereas fimE promotes recombination from ON to OFF. fimE is responsible for rapid recombination of the invertible element to the OFF orientation upon growth on solid media, suggesting that fimE plays an important regulatory role in phase variation. The second mode of phase variation is independent of recombination of the fim invertible element. Whereas strains in which the invertible element is locked OFF are afimbriate, strains locked ON (either by mutation of fimB and fimE, or by mutation of the invertible element) continue to exhibit phase-variable expression of type 1 fimbriae. Unlike on-off phase variation, the mechanism of inversion-independent phase variation is unclear. Neither fimB nor fimE has a dramatic effect on inversion-independent phase variation.