LOH Induction Mechanism of Saccharomyces cerevisiae.

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We examined canavanine-resistance (Can^R) mutations in a CAN1/ CAN1D::LEU2 heterozygote S. cerevisiae formed by mating UV-irradiated and unirradiated haploids. All the Can^R diploid cells from unirradiated haploid cells were found to be due to chromosome rearrangement or chromosome loss and Can^R-frequency was 0.4×10⁻⁴/cell. To see the effect of UV for induction of LOH, UV irradiated haploid cells with functional CAN1 gene were mated with unirradiated haploid cells with CAN1-deficent and Can^R diploid cells were selected. The frequency was 1.4×10⁻³/cell. After irradiation to CAN1-deficient haploid cell, Can^R frequency was 0.4×10⁻³/cell. Consequently, irradiation of functional CAN1 cells causes higher induction of LOH than irradiation of CAN1-deficent cells. Therefore, an existence of DNA lesions on the chromosome with functional CAN1 yields effective induction of LOH. By analysis of chromosome structure of Can^R diploid mutant, we observed that LOH of the CAN1 gene was mostly associated with replacement of the CAN1 gene to telomere with can1D to telomere of homologous chromosome. We argue that LOH could have occurred during the repair of DNA lesions on a damaged chromosome due to nonreciprocal recombination. [J Radiat Res 44:401 (2003)]