Control of recombination within and between DNA plasmids of Saccharomyces cerevisiae

[2 μm⁺ and [2μm°] yeast were transformed to stable leucine prototrophy with the hybrid yeast - E. coli plasmid, pJDB219. This plasmid contains the entire sequence of the endogenous 2 μm yeast DNA plasmid in addition to the yeast nuclear LEU2⁺ gene and the Co1E1 derivative, pMB9. In the [2 μm⁺] transformants, a new wholly yeast LEU2⁺ plasmid, pYX, was generated, probably by a recombination event between pJDB219 and 2 μm DNA. The plamid, pYX, in the absence of 2 μm DNA, was found to exist in equimolar amounts of two forms, A and B, which probably arise by intramolecular recombination across the inverted repeat sequences of the 2 μm DNA portion of the plasmid. pJDB219 was found to require the presence of 2 μm DNA to undergo this intramolecular recombination. The results suggest that 2, μm DNA and pYX code for a gene product required in this recombination event which pJDB219 cannot produce.