CONTROL OF ESCHERICHIA-COLI TYPE-1 FIMBRIAL GENE-EXPRESSION IN STATIONARY-PHASE - A NEGATIVE ROLE FOR RPOS

Expression of type 1 fimbriae in Escherichia coli is subject to phase- dependent control, with many regulatory inputs from proteins which org anise or rearrange the structure of DNA. Inversion of a DNA segment ca rrying the promoter for expression of fimA, the gene encoding the fimb rial subunit protein, makes a key contribution to the switching of cel ls between fimbriate and afimbriate states. We have discovered that tr anscription of fimA is repressed as cells enter stationary phase. This repression is not seen in isogenic strains deficient in rpoS, the gen e coding for the stationary phase-specific sigma factor RpoS. RpoS-def icient strains are also altered in the frequency of inversion of the f imA promoter segment. In the strains used in this study, inversion is catalysed by an integrase-like site-specific recombinase encoded by th e fimB gene. We report that fimB transcription is repressed strongly a s wild-type cells enter stationary phase but that this repression is a lleviated in cells deficient in RpoS. These data suggest that RpoS has a negative regulatory role which may be indirect, at both the fimA an d the fimB promoters.