Transposition of a tandem duplication of yeast mating-type genes

Abstract

The switching of mating-type genes in both homothallic and heterothallic strains of the yeast Saccharomyces cerevisiae involves a site-specific transposition event^1–3. A sequence at the mating-type locus (MAT) is replaced by a copy of a or α information from the unexpressed, loci, HML or HMR. Although MATa and MATα contain unique sequences of ∼700 base pairs (bp), they are flanked by sequences that are also found at HML and HMR^4,5 (Fig. 1a). Recent results^6–8 have suggested that the switching of MAT alleles involves an intrachromosomal mitotic gene conversion event, in which a donor sequence at HML or HMR pairs with the homologous sequences at the MAT locus. On this view the switching of mating-type information must be able to accommodate the region of non-homology in a heteroduplex structure. (Analogous gene conversions of non-homologous regions have been well documented during meiosis at MAT and for deletions in other yeast genes^9,10.) One prediction of a gene conversion mechanism is that it should be possible to transpose a tandem duplication of mating-type genes from HMR to MAT. In such a switch, pairing would occur between MAT and homologous sequences flanking the two copies of the tandem duplication at HMR. Our results reported here demonstrate that a tandem duplication created at HMR by recombinant DNA techniques can be transposed to MAT without the loss of these sequences at the donor locus.