INTERACTION OF FIMB AND FIME WITH THE FIM SWITCH THAT CONTROLS THE PHASE VARIATION OF TYPE-1 FIMBRIAE IN ESCHERICHIA-COLI K-12

The phase variation of type 1 fimbriae in Escherichia coil is associat ed with the site-specific inversion of a short DNA element. Recombinat ion at fim requires fimB and fimE, and their products are considered t o be the fim recombinases. In this study, FimB and FimE were overprodu ced and extracts containing the proteins were shown to (i) bind to and (ii) invert the firn switch in vitro. Phenanthroline-copper protectio n of DNA-protein complexes showed that both FimB and FimE bind to half -sites that flank, and overlap with, the left and right inverted repea ts (IRL and IRR, respectively) of the fim switch. Alignment of the fou r half-sites identified a conserved 5'-CA doublet; mutation of these t wo bases lowers the affinity of binding of both FimB and FimE to the i nverted repeats, and greatly diminishes inversion of the fim switch in vivo. The specificity of the fim recombinases observed in vivo (FimB switching in both directions; FimE switching from on-to-off only) was maintained in vitro. Furthermore, the different binding affinities of FimB and FimE for the various half-site combinations suggests that the specificity of FimE could arise, in part, from the low affinity of Fi mE for IRL (off).