Functional analysis of the FimE integrase of Escherichia coli K-12: isolation of mutant derivatives with altered DNA inversion preferences.

Phase variable expression of type 1 fimbriae in Escherichia coli arises from a site-specific recombination event that inverts a short segment of chromosomal DNA carrying the promoter for transcription of the gene encoding the fimbrial subunit protein. Two integrase-like recombinases are involved in switching. The FimB recombinase inverts the DNA segment in either orientation, whereas the FimE protein inverts it predominantly in the ON-to-OFF direction. In this paper, we report the isolation of a FimE mutant protein that has enhanced bidirectional switching activity. This protein has an arginine-to-lysine substitution at position 59, and this confers a FimB-like switching character on FimE without altering its ability to bind to DNA. The arginine was not a member of the arginine-histidine-arginine-tyrosine catalytic tetrad that is common to all integrase-like recombinases. The catalytic tetrad members of FimE were identified at positions 41, 136, 139 and 171 and shown to be essential for FimE function. In addition, other amino acid residues that make important contributions to the DNA binding activity of FimE or its ON-to-OFF inversion efficiency were identified.