The behavior of insertions near a site of mitotic gene conversion in yeast.

In yeast, coincident gene conversion events involving the LEU1 and TRP5 loci (16 cM apart) occur at frequencies that are far greater than is expected for two independent acts of recombination. When a large plasmid (pJM53) is placed between these genes so that a direct repeat is produced, there is frequent loss of the insert among coincident convertants. Previous results strongly suggest that this is due to a separate, intrachromosomal exchange between the direct repeats rather than to excision from an extensive region of heteroduplex DNA. In this paper, we extend our genetic and molecular analysis to a plasmid insertion (pKSH) which replaces rather than duplicates the chromosomal material. The relative stabilities of pKSH and pJM53 are compared among coincident Leu+Trp+ convertants and convertants involving only one locus (LEU1). The pKSH insertion is significantly more stable in the latter which constitute a large majority of the selectable recombinants. In the former, both insertions are lost with high frequency. These results are used to argue that, while most mitotic conversion does not result from long intermediates, coincident convertants may arise from either multiple intermediates or extensive heteroduplex regions.