INCREASED FREQUENCY OF UNEQUAL CROSSING-O VER IN CONJUGATIONAL CROSSES IN RECBC MUTANTS OF ESCHERICHIA-COLI

The formation of heterozygous tandem duplications was studied in conju gational crosses of Escherichia coli using mutations for deo-operon ge nes. The frequency of tandem duplications in recF::Tn3 and recBC sbcB mutants was not substantially changed from that of the wild type strai n. In contrast, the frequency of duplications was increased about ten- fold when using recBC (sbcB+) mutants as recipients. This result can b e explained by a participation of the RecBCD protein in the pairing of chromosomes during recombination. We also found that HfrH thyA recBC (sbcB+) bacteria grow very poorly on rich medium and that good growth is restored by conversion from the Hfr to the F+ state, as well as by mutations supressing the RecBC phenotype; such F+ strains retain sensi tivity to UV light and to mitomycin C, which are characteristic of the recBC phenotype.