Kirchmaier Ann L, Rine Jasper
Department of Biochemistry and Purdue Cancer Center, Purdue University, 175 S. University St., West Lafayette, IN 47907, USA.
Mol Cell Biol. 2006 Feb;26(3):852-62. doi: 10.1128/MCB.26.3.852-862.2006.
The establishment of silencing at the silent mating-type locus, HMR, in Saccharomyces cerevisiae requires that yeast pass through S phase of the cell cycle, yet requires neither the initiation of DNA replication at the locus destined to become silenced nor the passage of a replication fork through that locus. We tested whether this S-phase requirement reflects a window within the cell cycle permissive for recruitment of Sir proteins to HMR. The S-phase-restricted event necessary for silencing occurred after recruitment of Sir proteins to HMR. Moreover, cells arrested in early S phase formed silent chromatin at HMR, provided HMR was on a nonreplicating template. Replicating templates required a later step for silencing. These results provide temporal resolution of discrete steps in the formation of silent chromatin and suggest that more than one cell cycle-regulated event may be necessary for the establishment of silencing.
酿酒酵母中沉默交配型位点HMR处沉默状态的建立要求酵母经历细胞周期的S期，但既不需要在注定要沉默的位点起始DNA复制，也不需要复制叉通过该位点。我们测试了这种S期要求是否反映了细胞周期中允许Sir蛋白募集到HMR的一个窗口。沉默所需的S期受限事件发生在Sir蛋白募集到HMR之后。此外，只要HMR位于非复制模板上，停滞在早S期的细胞就能在HMR处形成沉默染色质。复制模板的沉默需要更晚的步骤。这些结果提供了沉默染色质形成过程中离散步骤的时间分辨率，并表明沉默的建立可能需要不止一个细胞周期调控事件。
