SUMMARY: A derepressed R factor, RY22, was transferred at a frequency of 5 X 10 between two strains of mated on a membrane. Under the same conditions transfer of this R factor from to was observed at a frequency of only 7.7 X 10. This frequency was greatly increased when the recipient strain was heat-treated before mating. Heat exposure for optimum fertility was 50 to 52 °C for a period of 2 to 3 min. Mutants of were isolated which were infected RY22 from or from at the same frequency. These observations strongly suggest that a DNA restriction and modification system in causes its low recipient ability for plasmids from other species.


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