Analysis of the fimB promoter region involved in type 1 pilus phase variation in Escherichia coli

Many strains of Escherichia coli express type 1 pili which undergo phase variation during growth in broth, but become uniformly nonpiliated when passaged on agar media. In a previous analysis of these agar phase locked strains, we demonstrated that fimB and fimE, which mediate the site-specific DNA rearrangement involved in phase variation, are differentially transcribed under the two growth conditions. In this study the fimB promoter region was sequenced and characterized in agar phase locked strain J96. Primer extension analysis of the fimB gene identified three putative transcription initiation sites. Transcription starting from two of the sites (P ₁ and P ₂ ) would produce an mRNA that approximates the transcript size of fimB previously detected by Northern blot hybridizations. Gel mobility shift studies revealed the presence of promoter binding activity in broth-grown cell extracts, but not in extracts from agargrown cells, that reacted specifically with DNA fragments upstream of the P ₁ and P ₂ promoter regions. This putative binding protein may be involved in the regulation of fimB perhaps by acting as a transcriptional activator.