Conjugal Donor Research Articles

High-frequency conjugal transfer of a gonococcal penicillinase plasmid.

Gonococci containing a 24 X 10(6)-dalton conjugal plasmid were able to mobilize for transfer a smaller, non-self-transmissible penicillinase (Pcr) plasmid with high frequency under appropriate conditions. In some strains, over 10% of donor colony-forming units transferred the Pcr plasmid in a mating of less than 2 h, which suggests that the conjugal system was naturally derepressed. Colony-opacity variants containing different quantities of an approximately 28,000-dalton outer membrane protein were altered in their ability to act as conjugal donors and recipients. Maximal transfer of the Pcr plasmid was observed between transparent donors and recipients, lacking appreciable amounts of the 28,000-dalton protein. Under conditions of high-frequency Pcr plasmid mobilization, no conjugal mobilization of chromosomal markers could be discerned.

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.