Integration host factor is required for the DNA inversion that controls phase variation in Escherichia coli.

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Abstract
The on-and-off expression (phase variation) of type 1 fimbriae, encoded by fimA, in Escherichia coli is controlled by the inversion of a promoter-containing 314-base-pair DNA element. This element is flanked on each side by a 9-base-pair inverted, repeat sequence and requires closely linked genes for inversion. Homology analysis of the products of these genes, fimB and fimE, reveals a strong similarity with the proposed DNA binding domain of .lambda. integrase, which mediates site-specific recombination in the presence of integration host factor. Integration host factor, encoded by himA and hip/himD, binds to the sequence 5'' TNYAANNNRTTGAT 3'', where Y = pyrimidine and R = purine, in mediating integration-excision. In analyzing the DNA flanking the fim 314-base-pair inversion sequence, we found the adjacent sequence 5'' TTTAACTTATTGAT 3'', which corresponds perfectly with the consensus integration host factor binding site. To characterize the role of himA in phase variation, we transduced either a deletion of himA or an insertionally inactivated hip/himD gene into an E. coli strain with a fimA-lacZ operon fusion. We found the rate of phase variation decreases sharply from 10-3 to < 10-5 per cell per generation. Southern hybridization analysis demonstrates that the himA mutation results in a failure of the switch-generated genetic rearrangement. When the transductant was trnsformed with a himA+ plasmid, normal switching returned. Thus integration host factor is required for normal type 1 fimbriae phase variation in E. coli.