Kirchmaier A L, Rine J
Division of Genetics and Development, Department of Molecular and Cell Biology, University of California, 401 Barker Hall, Berkeley, CA 94720, USA.
Science. 2001 Jan 26;291(5504):646-50. doi: 10.1126/science.291.5504.646.
In Saccharomyces cerevisiae, the silent mating loci are repressed by their assembly into heterochromatin. The formation of this heterochromatin requires a cell cycle event that occurs between early S phase and G(2)/M phase, which has been widely assumed to be DNA replication. To determine whether DNA replication through a silent mating-type locus, HMRa, is required for silencing to be established, we monitored heterochromatin formation at HMRa on a chromosome and on a nonreplicating extrachromosomal cassette as cells passed through S phase. Cells that passed through S phase established silencing at both the chromosomal HMRa locus and the extrachromosomal HMRa locus with equal efficiency. Thus, in contrast to the prevailing view, the establishment of silencing occurred in the absence of passage of the DNA replication fork through or near the HMR locus, but retained a cell cycle dependence.
在酿酒酵母中，沉默的交配型基因座通过组装成异染色质而受到抑制。这种异染色质的形成需要一个发生在S期早期和G(2)/M期之间的细胞周期事件，人们普遍认为这一事件是DNA复制。为了确定通过沉默交配型基因座HMRa的DNA复制对于建立沉默是否必要，我们在细胞进入S期时监测了染色体上和非复制型染色体外载体上HMRa处的异染色质形成。通过S期的细胞在染色体HMRa基因座和染色体外HMRa基因座上以相同效率建立了沉默。因此，与普遍观点相反，沉默的建立发生在DNA复制叉未通过或靠近HMR基因座的情况下，但仍保留细胞周期依赖性。
