Asymmetric effects of deletions and substitutions on high negative interference in coliphage lambda

Experiments have been performed to help clarify the role of nonhomologies in phage lambda recombination. Three-factor crosses were carried out, and the frequencies of single and double recombinants in the two adjoining intervals were compared when the central marker was either a double point mutation (v1v3) or deletion (rex-cI deletion) or nonhomologous substitution (imm434). In all cases the lefthand marker was a bio substitution (Fec- phenotype, which does not permit plating on recA-), and the righthand marker was an amber mutation in gene O. Experiments were performed in all four possible arrangements of the central and rightward markers, while selecting for the Fec+ phenotype on the recA- host. As anticipated, high negative interference (HNI) was observed with point mutations, but when the central marker was a substitution nonhomology, HNI was reduced about tenfold. Surprisingly, when the central marker was a simple deletion, a dramatic asymmetry in results was observed, with HNI being exhibited only when the central deletion marker was acquired by the double recombinant. These results indicate that under normal conditions (red+, gam+, rec+) and with noninhibited DNA replication, recombination in coliphage lambda entails a highly asymmetric step that could be at the level of strand transfer or mismatch repair.