THE SITE-SPECIFIC RECOMBINATION SYSTEM REGULATING EXPRESSION OF THE TYPE-1 FIMBRIAL SUBUNIT GENE OF ESCHERICHIA-COLI IS SENSITIVE TO CHANGES IN DNA SUPERCOILING

We have studied the effect of altering the in vivo level of DNA superc oiling on the phase-variable expression of the Escherichia coil fimA g ene. Transcription from the fimA promoter was unaffected by changes in DNA supercoiling whether caused by the introduction of a topA::Tn10 m utation or by inhibition of DNA gyrase with the antibiotic novobiocin. However, inversion of the fimA promoter fragment was altered in respo nse to perturbation of DNA supercoiling. Specifically, inactivation of topA reduced the rate of promoter fragment inversion in both the ON-t o-OFF and the OFF-to-ON directions. This effect correlated with the lo ss of functional topA and not with the global level of DNA supercoilin g. Inhibition of DNA gyrase introduced a bias in favour of the OFF-to- ON inversion; the ON-to-OFF inversion was affected only slightly. Chan ges in expression of fimB, the gene coding for the recombinase that ca talyses fimA promoter fragment inversion in the strains used in this s tudy, did not correlate with effects on fimA phase variation: we found that transcription of fimB was inhibited by loss of functional topA a nd was enhanced by inhibition of DNA gyrase in a manner that correlate d well with the global level of in vivo DNA supercoiling. A model is p resented to account for the effects of lost topoisomerase function on fimA gene expression.