The addition of lac+ chromosome fragments to the E. coli proA-proB-lac deletion 13 chromosome.

Escherichia coli with the proA-proB-lac deletion X111 (Delta111) can be transduced with bacteriophage P1 propagated on a wild-type lac(+) donor. Though the donor lac(+) genes cannot be integrated by replacement of the recipient Delta111 marker, the transduction process has the characteristics generally associated with generalized transduction of bacterial genes. Transduction does not require P1 helper infection, is stimulated by UV irradiation of transducing particles, and does require homology between the donor lac(+) chromosome and the recipient Delta111 chromosome. Among transductants produced through multiple P1 infection, a minority of P1dl lysogens are present. But the majority of the transductants have unstable lac(+) units, designated lacV, which are without detected P1 gene content. LacV is tightly linked to the Delta111 locus. Instability of lac(+) is eliminated when a recombination deficiency is introduced through a substitution of recA1 for rec(+). The properties of the Delta111/lacV strains are attributable to a chromosome in which lac(+) is situated between units of a genetic duplication beside the Delta111 locus. To explain the formation of partially diploid chromosomes we suggest that chromosome fragment integration is sometimes accomplished through a single aberrant recombination, a fusion of genetically heterologous DNA ends, and a single legitimate crossover.