Effect of donor copy number on the rate of gene conversion in the yeast Saccharomyces cerevisiae.

Nonreciprocal recombination (gene conversion) between homologous sequences at nonhomologous locations in the genome occurs readily in the yeast Saccharomyces cerevisiae. In order to test whether the rate of gene conversion is dependent on the number of homologous copies available in the cell to act as donors of information, the level of conversion of a defined allele was measured in strains carrying plasmids containing homologous sequences. The level of recombination was elevated in a strain carrying multiple copies of the plasmid, compared with the same strain carrying a single copy of the homologous sequences either on a plasmid or integrated in the genome. Thus, the level of conversion is proportional to the number of copies of donor sequences present in the cell. We discuss these results within the framework of currently favoured models of recombination.