Recent work has demonstrated that expression of type 1 fimbriae is repressed by
PapB, a regulator of
pyelonephritis-associated pili (P-pili).
PapB belongs to family of related adhesin regulators, for which consensus residues required for
DNA binding and oligomerization have been identified. Of the regulators tested in this study,
PapB, SfaB (S-fimbriae) and PefB (Salmonella enterica serovar Typhimurium--plasmid-encoded fimbriae) repressed FimB-promoted off-to-on inversion of the fim switch, although complete repression was only demonstrated by
PapB. DaaA, FaeB, FanA, FanB and ClpB had no effect on fim switching. In addition, only
PapB stimulated FimE-promoted on-to-off inversion. Deletion analysis demonstrated that this specificity resides in the carboxy terminal of the
protein, and not the amino terminal, with the central region being homologous among the family members. Exchange of Leu(82) and Ile(83) of
PapB for the equivalent residues from the DaaA
protein (Phe and Gln) within the carboxy terminal virtually abolished cross-talk activity. Whereas
PapB can bind to a region around the left inverted repeat of the fim switch, DaaA and the
PapB double mutant were effectively unable to bind this region. A previously characterized
PapB DNA binding mutant also failed to bind to this region and failed to inhibit FimB activity at the fim switch. Thus, repression of fim expression appears unique to
PapB and SfaB within E. coli and requires
DNA binding involving
amino acid residues located both within the homologous core and in the heterogeneous carboxy terminus. The variation in the carboxy terminus between the
PapB family members explains their differential effects on fim. This mechanism of cross-talk seems restricted to the P and S family adhesins with type 1 fimbriae and may ensure variable and sequential expression of adhesins during
urinary tract infections.