A dherence to host cells relies on the synthesis of specialized molecules that play an important role in multiple steps during the infectious process. Adhesins contribute to virulence by promoting the attachment and bacterial colonization on the host cell surface. Escherichia coli possesses a wide variety of adhesins, which all differ in structure and in functions. Moreover, a single strain can present more than one adhesin. This implies a complex regulatory network to produce exclusively one adhesin or to simultaneously express more than one.Phase variation of P fimbriae is a stochastic and reversible switch between an on phenotype and an off phenotype, "all-ornothing" (ON/OFF), each capable of responding to a specific condition. After cell division, the majority of daughter cells retain the expression phase of the parent; a minority switches to the opposite expression phase. F165 1 fimbriae, encoded by the foo gene cluster, are members of the type P fimbria family of Escherichia coli (1-4). The regulatory region of foo shares more than 96% identity with that of the pyelonephritis-associated pili (pap) gene. Previous work showed that F165 1 fimbriae are essential for virulence and survival during systemic stages of infection in young pigs (5-8). F165 1 fimbriae are principally found in infections caused by opportunistic E. coli in animals, while Pap fimbriae are found in human clinical cases of urinary tract infections. The phase variation of F165 1 fimbriae shares similar features with that of the wellcharacterized Pap fimbriae (9-11). This includes an ϳ400-bp intergenic region surrounded by two divergently transcribed genes (papI and papB for Pap, fooI and fooB for F165 1 ). It also includes six leucine-responsive regulatory protein (Lrp) binding sites, of which two are GATC sites (referred to as GATC prox and GATC dist , respectively), spaced 102 bp apart. In pap and foo, phase variation occurs because of methylation-controlled modifications within the regulatory region of the fimbrial operon. This regulation is epigenetic since it does not involve any changes in DNA sequence (12). The differential methylation status of these sequences determines the binding of Lrp. Indeed, the switch from one phenotype to the other arises from the competition between the binding of Lrp at its distal or proximal binding sites and methylation by the Dam methyltransferase at the opposite GATC site (for reviews, see references 9, 10, and 11). When GATC dist is fully methylated, Lrp cooperatively binds to sites 1 to 3, maintaining the cells in the OFF state; when GATC prox is fully methylated, Lrp binds to sites 4 to 6, maintaining the ON state (Fig. 1). Control of pap expression also requires the action of PapI, a positive regulator that increases the affinity of Lrp for sites 4 to 6 in vivo (11,13). PapB, the second specific regulator of the pap operon, plays an important role by coordinating the expression of the pBA and pI promoters (11). All together, the combined actions of Lrp, Dam methyltransferase, PapI, and PapB result...

c CS31A, a K88-related surface antigen specified by the clp operon, is a member of the type P family of adhesive factors and plays a key role in the establishment of disease caused by septicemic and enterotoxigenic Escherichia coli strains. Its expression is under the control of methylation-dependent transcriptional regulation, for which the leucine-responsive regulatory protein (Lrp) is essential. CS31A is preferentially in the OFF state and exhibits distinct regulatory features compared to the regulation of other P family members. In the present study, surface plasmon resonance and DNase I protection assays showed that Lrp binds to the distal moiety of the clp regulatory region with low micromolar affinity compared to its binding to the proximal moiety, which exhibits stronger, nanomolar affinity. The complex formation was also influenced by the addition of PapI or FooI, which increased the affinity of Lrp for the clp distal and proximal regions and was required to induce phase variation. The influence of PapI or FooI, however, was predominantly associated with a more complete shutdown of clp expression, in contrast to what has previously been observed with AfaF (a PapI ortholog). Taken together, these results suggest that the preferential OFF state observed in CS31A cells is mainly due to the weak interaction of the leucine-responsive regulatory protein with the clp distal region and that the PapI homolog favors the OFF phase. Within the large repertoire of fimbrial variants in the P family, our study illustrates that having a fimbrial operon that lacks its own PapI ortholog allows it to be more flexibly regulated by other orthologs in the cell.