DIFFERENTIAL TEMPERATURE MODULATION BY H-NS OF THE FIMB AND FIME RECOMBINASE GENES WHICH CONTROL THE ORIENTATION OF THE TYPE-1 FIMBRIAL PHASE SWITCH

Phase variation of type I fimbriation in Escherichia coli is associate d with the inversion of a 314-bp DNA element. This DNA switch directs transcription of fimA, encoding the major type 1 fimbrial subunit, ill the on orientation but not in the off orientation. Inversion of the D NA element requires either FimB (confers both on and off orientations) or FimE (confers off orientation). Here we show, by chromosomally loc ated fimB- and fimE-lacZ cassettes in isogenic strain sets differing o nly in the hns locus, how the global regulator H-NS affects the expres sion of type I fimbriae. H-NS was found to downregulate fimB and fimE in a temperature-dependent manner which affected the genes inversely a t 30 degrees C and 37 degrees C. By gel-retardation assays H-NS bindin g was demonstrated to the regions containing the fimB promoter and the fimE promoter, respectively. In vitro recombination analysis suggeste d no direct involvement of H-NS in the inversion of the phase switch. Rather than directly affecting the switching process per se, it appeal ed that the orientation of this element was affected by the different ial temperature modulation of H-NS of the fimB and fimE genes. Taken t ogether the results suggest that H-NS modulates expression of type 1 f imbriae in a way which seems to favor a fimbriate stale at the, mammal ian body temperature. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.