Very short patch mismatch repair in phage lambda: repair sites and length of repair tracts.

Five amber mutations in the repressor (cI) gene of bacteriophage lambda recombine anomalously with nearby cI mutations. When any of these markers is used in four-factor crosses, cI+ recombinants that are expected to require three cross-overs occur at high frequencies. These recombinants are attributable to very-short-patch (VSP) repair of specific mismatches in DNA heteroduplexes formed during recombination between the markers flanking cI. The sites of the repair-prone mutations and the lengths of repair tracts have now been determined. Amber mutations subject to VSP repair are C to T transitions in 5'CCATGG, the sequence methylated by the product of gene dcm, and also in the related 5'CAGG or 5'CCAG sequences. Ambers arising in CAG sequences found in other contexts, or in codons other than CAG, were not subject to VSP repair. Repair tracts rarely, if ever, exceed ten nucleotides in length, and can be as short as two nucleotides. A repair-prone mutation does not stimulate recombination between flanking cI markers.