Copyright © 1992, American Society for Microbiology Crossover Site Selection during Recombination of

Two hybrid replicons containing polyomavirus (Py) genomes with large duplications of the viral late coding sequences were transfected into various permissive mouse cell lines. In all cell lines, either replicon yielded the sole amplifiable product expected from intramolecular homologous recombination, unit-length Py DNA (P155). In normal and in Py-transformed cells, such recombination was highly effective and involved sequences previously found to act as recombination hot spots (S repeats). In cells transformed by simian virus 40, however, these hot spots were inoperative in the generation of P155, which occurred with a reduced efficiency. These data confirm and extend earlier data indicating that the nature of products arising from recombination in Py replicons is tightly controlled by both cis- and trans-acting factors. To probe the recombinational mechanisms of cultured mouse cells, we have been using RmI, a polyomavirus (Py)-mouse hybrid replicon which includes 1.6 kb of mouse DNA inserted between two 182-bp direct viral repeats (S repeats) in an otherwise unrearranged Py genome (Fig. 1). When RmI is transfected into normal or Py-transformed mouse cells, high-frequency crossing over takes place be