A reanalysis of red-mediated reciprocal recombination in bacteriophage lambda

Circular dimers of a lambda DNA in infected, nonlysogenic Escherichia coli

Covalently closed circular dimers of phage lambda DNA have been found in Escherichia coli infected with lambda. These dimers can be formed by either the lambda Red or Int systems, by a nonrecombinational replicative mechanism requiring the activity of the lambda O and P genes or by joining of the cohesive ends. Dimers mediated by the E. coli Rec system have not been observed. Those formed by the Int system often result from recombination between different DNA molecules; however, the Red-mediated dimers may be a result of replicative extension of a single DNA molecule. Trimers have also been observed but studied only briefly.

Single burst study of rec- and red-mediated recombination in bacteriophage lambda

Single bursts from three-point crosses of bacteriophage lambda were analyzed for recombination of markers several thousand nucleotide pairs apart. Single recombination, mediated by the rec system of Escherichia coli, is usually reciprocal. Double recombinants are also significantly correlated in single bursts, although the correlation is weaker than for reciprocal singles. Reciprocity is not found in crosses mediated by the lambda red system. With respect to certain other paraments of recombination, the two systems appear to be alike. Double recombinants usually result from one event, not from two independent single recombinations, and their average clone size is about half that of single recombinants. The results are discussed in terms of current molecular models of recombination.