Mapability of very close markers of bacteriophage lambda.

Recombinant frequency was compared with nucleotide distance in crosses involving markers in either the PRM or the cy region of phage lambda. For each pair of markers, we performed reciprocal four-factor crosses of the following types: (I) A+m1+m2-B- X A-m1-m2+B+; and (II) A+m1-m2+B- X A-m1+m2-B+. In crosses of type I, the frequency of A+m1+m2+B+ recombinants among total (selected) A+B+ progeny was directly proportional to nucleotide distance between m1 and m2 in the range from 3 to 160 nucleotides. When less than three nucleotides separated m1 and m2, the measured yields of m1+m2+ recombinants were significantly depressed. We also found that the frequency of A+m1+m2+B+ recombinants among total A+B+ progeny was significantly lower (about 10-fold on the average) in crosses of type II than in the corresponding crosses of type I. Since mismatch correction should yield A+m1+m2+B+ recombinants with approximately equal frequencies in type I and II crosses, we suggest: (1) that most m1+m2+ recombinants produced in type I crosses must arise from the formation of heteroduplex structures with a discontinuity (in the source of genetic information) between sites m1 and m2, and (2) that mismatch correction is not a major pathway for production of recombinants for close markers in normal lambda infection.