Chromosomal mutations of Escherichia coli that alter expression of conjugative plasmid functions (cpx genes/F-pili/conjugal donor activity/surface exclusion/outer membrane)

We have identified two chromosomal genes of Escherichia coli K12 that are required for the expression of coniugative plasmid functions in the presence of normal plas- mid DNA. Hfr cells with mutations in both of these genes are resistant to donor-specific bacteriophage and defective as conjugal donors. These characteristics can be attributed to the inability of mutant Hfr cells to elaborate F-pili, surface organ- elles required both for conjugal donor ability and for sensitivity to donor-specific bacteriophages. Mutant cells are also defective in surface exclusion, the property of donor cells to act as poor conjugal recipients. This defect can be attributed in part to a reduction in the amount of the F-plasmid traTgene product in the outer membrane of mutant cells; this protein is one of two plasmid gene products required for the full expression of surface exclusion. We have designated the chromosomal genes identi- fied by these mutations as cpxA and cpxB; the mnemonic cpx signifying conjugative plasmid expression.