F42 lac recombination with λplac5is normally twentyfold to fiftyfold higher than recombination between λplac5 and a chromosomal lac gene. The presence of an fi+ R1 plasmid in the same cells as F42 lac dramatically reduces this enhanced recombination level while the fi- R1 drd19 plasmid has little effect. When F42 lac traJ90 is tested in a sup+ strain, it shows a sharp reduction in recombination with λplac5 that can be largely reversed by the presence of a supF mutation that partially suppresses the traJ90 nonsense mutation. It is concluded that the enhanced recombination between F42 lac and λplac5 is largely dependent on the constitutive expression of F42 lac fertility functions.
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