ORIENTATION-DEPENDENT ENHANCEMENT BY H-NS OF THE ACTIVITY OF THE TYPE-1 FIMBRIAL PHASE SWITCH PROMOTER IN ESCHERICHIA-COLI

Phase variation of type 1 fimbriation in Escherichia coli is associate d with the inversion of a 314-bp DNA element, positioned proximal to a nd upstream of fimA, which encodes the major type 1 fimbrial subunit. This DNA switch region contains a promoter that drives transcription o f fimA only when the switch is in the ON orientation. Using chromosoma l and plasmid-borne lacZ reporter cassettes, we show here how the glob al regulator H-NS affects the activity of the fimA promoter. In phase- locked reporter cassettes the activity of the fimA promoter was found to be enhanced in a hns-positive background, but only when the switch was in the ON orientation. Also, the number of fimbriae produced by a phase-locked ON strain was significantly higher in a hns-positive back ground. By means of competitive gel retardation and DNase I protection assays, H-NS binding to DNA segments adjacent to and within the phase switch region was demonstrated.