Figure 1. Phase variation of type 1 fimbriae expression in E. coli.

Type 1 fimbriae phase variation is controlled by the invertible DNA element, fimS, which contains the promoter for the genes encoding structural fimbriae subunits (including fimA and fimH) and is flanked by two inverted repeat sequences: IRL and IRR. (In this diagram, IRL is the inverted version of IRR.) When the switch is in the ON position, transcription of structural fim genes can be initiated because the promoter is in the appropriate orientation. However, when the switch is inverted into the OFF position, the promoter points in the opposite direction and so no longer supports the expression of fimbriae components—leading to their rapid degradation. The ON-to-OFF inversion of the switch is mediated by recombinases FimE and FimB, while the OFF-to-ON events are mediated by FimB.