Studies on mitotic gene conversion inUstilago

In order to develop a system for the study of the mechanism of intragenic recombination in Ustilago , mutants lacking nitrate reductase activity were isolated, and five alleles were combined in pairs in ten vegetative heteroallelic diploids. The diploids have the mutant phenotype, i.e. inability to utilize nitrate as sole source of nitrogen, but they will recombine to produce wild-type cells much more frequently than the back-mutation rates of haploids or homoallelic diploids. The spontaneous rate of recombination can be enormously increased by low doses of UV light, particularly if treatment is during the period of DNA synthesis in the mitotic cycle. By means of half-tetrad analysis it has been shown that this process of intragenic recombination, as in other fungi, is due to gene conversion rather than reciprocal exchange. It has also been shown that the frequency of UV-induced conversion under standard conditions gives a rough measure of the distance between two mutant sites, since it was possible to use these frequencies to make a linear fine structure map of the gene. These results are discussed in relation to a hybrid DNA model for gene conversion slightly modified from that previously suggested for meiotic recombination.