Chromosomal mutations of Escherichia coli that alter expression of conjugative plasmid functions

We have identified two chromosomal genes of Escherichia coli K12 that are required for the expression of conjugative plasmid functions in the presence of normal plasmid 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 organelles 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 traT gene 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 identified by these mutations as cpxA and cpxB; the mnemonic cpx signifying conjugative plasmid expression.