Jensen R, Sprague G F, Herskowitz I
Proc Natl Acad Sci U S A. 1983 May;80(10):3035-9. doi: 10.1073/pnas.80.10.3035.
The ultimate product of yeast mating-type interconversion is a stable a/alpha diploid cell. A haploid cell carrying the HO gene gives rise to a diploid cell in a two-step process: first, the cell switches mating type as a result of genetic rearrangement (cassette substitution) catalyzed by HO; then, cells of opposite type mate to form a/alpha diploids. Mating-type interconversion does not occur in a/alpha diploids despite the presence of the HO gene. We have identified a plasmid carrying the HO gene by screening a yeast clone bank (constructed in vector YEp13) for plasmids that allow mating-type switching by ho cells. The yeast segment responsible for mating-type interconversion integrates by homology at the ho locus, thus confirming that it carries HO. Using the HO gene as a probe, we find that strains with an active mating-type interconversion system produce HO RNA, whereas a/alpha HO/HO cells do not and that this inhibition requires products of both the MATa1 and MATa2 genes. Thus, mating-type interconversion does not occur in a/alpha HO/HO cells because the HO gene product is not synthesized. These results demonstrate the following: (i) The mating-type locus, proposed on genetic grounds to be a regulatory locus, controls expression of an unlinked gene (HO) at the level of RNA production. (ii) The HO gene is under negative feedback control: its expression is inhibited after successful completion of diploidization (formation of a/alpha diploids).
酵母交配型相互转换的最终产物是稳定的a/α二倍体细胞。携带HO基因的单倍体细胞通过两步过程产生二倍体细胞：首先，由于HO催化的基因重排（盒式替换），细胞转换交配型；然后，相反类型的细胞交配形成a/α二倍体。尽管存在HO基因，但交配型相互转换在a/α二倍体中并不发生。我们通过筛选酵母克隆文库（构建于载体YEp13中）寻找能使ho细胞发生交配型转换的质粒，从而鉴定出一个携带HO基因的质粒。负责交配型相互转换的酵母片段通过同源性整合到ho位点，从而证实它携带HO。用HO基因作为探针，我们发现具有活跃交配型相互转换系统的菌株产生HO RNA，而a/α HO/HO细胞不产生，并且这种抑制需要MATa1和MATa2基因的产物。因此，交配型相互转换在a/α HO/HO细胞中不发生是因为没有合成HO基因产物。这些结果表明：（i）基于遗传学理由提出的交配型位点作为一个调控位点，在RNA产生水平上控制一个不连锁基因（HO）的表达。（ii）HO基因受到负反馈控制：在二倍体化（形成a/α二倍体）成功完成后，其表达受到抑制。
