Isolation and identification of fxsA, an Escherichia coli gene that can suppress F exclusion of bacteriophage T7.

A selection for mutants of Escherichia coli that survive coexpression of bacteriophage T7 gene 10 and plasmid F pifA has allowed the identification of a newly defined genetic locus, fxsA. fxsA is located at 94.1 min on the E. coli chromosome; the gene is monocistronic and non-essential for growth. Overexpression of fxsA is necessary for resistance to the toxicity of T7 gene 10 in the presence of pifA; the original mutant strain contains a promoter-up mutation, changing a G residue to the "invariant" T in the -10 hexamer of a sigma(70)promoter. This chromosomal mutation causes a 25-fold increase in the level of fxsA mRNA. The initiation codon of fxsA is shown to be UUG, and the FxsA protein is then deduced to consist of 158 amino acid residues. A similar mutant selection that demanded cell survival to a challenge of T7 gene 1.2 and pifA also resulted in the isolation of the identical promoter-up mutation that affects expression of fxsA. The increased levels of FxsA resulting from the promoter-up mutation allow phage T7 to avoid exclusion by the F plasmid, presumably by protecting the cell from premature death due to gene 10 or to gene 1.2 expression in the presence of the PifA protein. Copyright 1999 Academic Press.