Haploid Saccharomyces cells have the remarkable potential to change mating type as often as every generation, a process accomplished by an intrachromosomal gene conversion between an expressor locus MAT and one of two repositories of mating type information, HML or HMR. The particular locus selected as donor is dictated by the mating type of the cell, a bias that ensures productive mating type interconversion. Here we use green¯uorescent protein tagging of the expressor and donor loci on chromosome III to show that this preference for donor locus does not result from a predetermined organization of chromosome III: HML and MAT as well as HMR and MAT remain separated in cells of both mating types. In fact, cells in which the inappropriate donor locus is arti®cially tethered to MAT still predominantly select the correct donor. We ®nd, though, that initiation of switching leads to a rapid association of the correct donor locus with MAT. Thus, in mating type switching in Saccharomyces, donor preference is imposed at commitment to recombination rather than at physical contact of interacting DNA strands.

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Haploid Saccharomyces can change mating type through HO-endonuclease cleavage of an expressor locus, MAT, followed by gene conversion using one of two repository loci, HML or HMR, as donor. The mating type of a cell dictates which repository locus is used as donor, with a cells using HML and ␣ cells using HMR. This preference is established in part by RE, a locus on the left arm of chromosome III that activates the surrounding region, including HML, for recombination in a cells, an activity suppressed by ␣2 protein in ␣ cells. We have examined the ability of RE to stimulate different forms of interchromosomal recombination. We found that RE exerted an effect on interchromosomal mating-type switching and on intrachromosomal homologous recombination but not on interchromosomal homologous recombination. Also, even in the absence of RE, MAT␣ still influenced donor preference in interchromosomal mating-type switching, supporting a role of ␣2 in donor preference independent of RE. These results suggest a model in which RE affects competition between productive and nonproductive recombination outcomes. In interchromosome gene conversion, RE enhances both productive and nonproductive pathways, whereas in intrachromosomal gene conversion and mating-type switching, RE enhances only the productive pathway.H APLOID Saccharomyces cells have the remarkchromosome III. Namely, mothers, but not daughters, able potential to change mating type as often as transcribe the HO gene and do so only during the G 1 every generation (reviewed in Haber 1998; Bi and phase of the cell cycle. Accordingly, only mothers are Broach 1999). The mating type of a haploid cell is capable of switching cell type and the switch occurs prior dictated by the particular allele, a or ␣, present at the to DNA replication in the mother (Nasmyth 1993; mating-type locus, MAT, located near the center of chroLong et al. 1997; Nasmyth and Jansen 1997). In addimosome III. Mating-type switching is initiated by a doution, cell type dictates which donor locus is selected ble-strand break at the MAT locus, catalyzed by an enfor participation in the gene conversion event. a cells donuclease encoded by HO (Strathern et al. 1982).predominantly select HML, which normally contains ␣ Switching then occurs by a gene conversion event that mating information, whereas ␣ cells select HMR, which replaces the mating information at the MAT locus with normally contains a mating information (Figure 1, botthe opposite mating information present at either of tom). This pattern ensures that most of the switching two repository mating loci, HML and HMR, located at events result in a change of mating type, rather than a the opposite ends of chromosome III (180 and 90 kb, futile replacement of the MAT allele with the same respectively, away from MAT). This results in replacesequence . ment of one mating-type allele at MAT with a copy ofThe fact that cells can select between HML and HMR the opposite mating-type allele taken from either HML loci implies that the two loci possess distin...