COINCIDENT GENE CONVERSION EVENTS IN YEAST THAT INVOLVE A LARGE INSERTION

John E. Golin ¹, S. Carl Falco ¹, and Jeanne P. Margolskee ²

¹ Central Research and Development, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19898
² Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143

In yeast, spontaneous gene conversion events involving sites that are far apart (16 cM) occur 1000 times more frequently in mitotic cells than is expected for two independent acts of recombination. It has been proposed that a major portion of these could be due to a long, continuous heteroduplex intermediate. We have examined this possibility in further detail by introducing, via transformation, a large plasmid insertion between the LEU1 and TRP5 loci and studying its behavior among coincident convertants involving the flanking sites. Among such convertants, there is frequent loss of the plasmid when it is present in hemizygous or homozygous configuration. Our results could support the long heteroduplex model for coincident recombination events, but only if novel assumptions regarding the formation and fate of mismatched DNA are made. Therefore, an alternative model that proposes multiple, concerted recombination events is discussed.

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