Regulation of the phase switch controlling expression of type 1 fimbriae in Escherichia coli.

The expression of Escherichia coli type 1 fimbriae is phase-variable i.e. the bacterial cell is either fimbriated or non-fimbriated. The transition from one state to the other is caused by the change in configuration of an invertible DNA segment harbouring the promoter of the fimA gene. The position of this phase switch is controlled by two proteins, FimB and FimE, which mediate an 'on' or 'off' configuration of the switch, respectively. In this study, we have investigated how these proteins control the switch by means of fim-lac fusions on low-copy-number plasmids. It was found, by in trans and cis complementation, that the ratio of fimB to fimE and the total concentration of the gene products determine the configuration of the switch as well as the frequency of phase switching. It was also shown that transcription occurs from the promoter located at the phase switch when this is in the 'off' configuration. This suggests a regulatory mechanism, since the resulting transcript would be anti-sense to the fimE transcript.