Alternative Homologous andNonhomologous Products Arising fromIntramolecular Recombination

RmI,achimeric DNA molecule containing polyomavirus (Py) andmouse sequences,generates unit-length Py DNA viaintramolecular recombination between twodirectly repeated viral sequencesof182basepairs (S repeats). Toanalyze thecontribution oftheSrepeats inthis process,we produced mutantsofRmIcarrying deletions ineither one or bothSrepeats andtested themfortheir ability torecombine inmouse 3T6cells. MutantDNAswere foundtoyield unit-length PyDNA aslong astheycarried a minimal internal homology of 40to50basepairs. Unlike RmIitself, however, themutantsalso gaverise tononhomologous recombination products. Theseresults suggest thatwhenthegeneration ofhomologous products ishampered bya limiting homology, nonhomologous products may ariseinstead ofhomologous ones.Therefore, theinitial step(s) inthe mechanisms yielding thetwokinds ofproducts could beidentical.