Multicopy fimB gene expression in Escherichia coli: Binding to inverted repeats in vivo, effect on fimA gene transcription and DNA inversion

Transcription of fimA, the Escherichia coli gene encoding the type 1 fimbrial subunit protein, is driven by a promoter carried on a 314 bp segment of invertible DNA. We have discovered that overexpression of fimB, one of the genes required for inversion of this DNA element, results in transcriptional repression of fimA. Furthermore, under these conditions inversion ceases to be dependent on the integration host factor (IHF) or the leucine-responsive regulatory protein (LRP), cofactors hitherto considered to be essential for inversion. Inversion will even occur (albeit at a very low level) in the absence of both cofactors, The interaction of the fimB gene product with the invertible element was studied in vivo in the presence of single- and multicopy fimB genes, Dimethyl sulphoxide (DMS)-mediated methylation of DNA at the 9 bp inverted repeats, which flank the invertible element, was found to vary in the presence and absence of functional fimB. The DMS reactivity profile at the left-hand inverted repeat was similar with single or multicopy fimB. The corresponding profile at the right-hand inverted repeat varied with fimB copy number. As this repeat lies between the fimA promoter and open reading frame, FimB binding here is likely to modulate fimA transcription and vice versa.