Pap pili play an important role in the pathogenesis of upper urinary tract infections by enabling uropathogenic Escherichia coli to adhere to host epithelial cells. Pap pili are coded for by the pyelonephritis-associated pili (pap) operon, which consists of 11 genes required for the expression and assembly of Pap pili. Expression of Pap pili is regulated by a phase variation mechanism in which the pili expression state of the bacterial population is skewed between phase-on (expression positive) and phase-off (expression negative) states. Pap phase variation is controlled by the cooperative binding of leucine-responsive regulatory protein (Lrp) to two sets of Lrp binding sites in the upstream regulatory region of the pap operon. A single GATC sequence, which is the target site for deoxyadenosine methylase (Dam), is centrally located within each Lrp binding region. Dam plays a critical role in the expression of Pap pili via the formation of DNA methylation patterns. Methylation of GATC-I reduced the affinity of Lrp for pap DNA by about twofold. Conversely, Lrp specifically blocked methylation of pap GATC-I in vitro. These data support the hypothesis that Lrp and Dam compete for binding to GATC-I, and are consistent with previous results indicating that methylation of GATC-I is important for stability of the phase-off state.
Copyright 1998 Academic Press.