[Increased frequency of unequal crossing over in recBC mutant cells in conjugational crosses in Escherichia coli K-12].

The formation of heterozygous tandem duplications was studied in conjugational crosses of Escherichia coli using mutations for deo-operon genes. The frequency of tandem duplications in recF::Tn3 and recBC sbcB mutants was not substantially changed from that of the wild type strain. In contrast, the frequency of duplications was increased about ten-fold when using recBC (sbcB+) mutants as recipients. This result can be 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 suppressing the RecBC phenotype; such F+ strains retain sensitivity to UV light and to mitomycin C, which are characteristic of the recBC phenotype.