Weiss K, Simpson R T
Department of Biochemistry and Molecular Biology, The Center for Gene Regulation, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Mol Cell Biol. 1998 Sep;18(9):5392-403. doi: 10.1128/MCB.18.9.5392.
Genetic studies have suggested that chromatin structure is involved in repression of the silent mating type loci in Saccharomyces cerevisiae. Chromatin mapping at nucleotide resolution of the transcriptionally silent HMLalpha and the active MATalpha shows that unique organized chromatin structure characterizes the silent state of HMLalpha. Precisely positioned nucleosomes abutting the silencers extend over the alpha1 and alpha2 coding regions. The HO endonuclease recognition site, nuclease hypersensitive at MATalpha, is protected at HMLalpha. Although two precisely positioned nucleosomes incorporate transcription start sites at HMLalpha, the promoter region of the alpha1 and alpha2 genes is nucleosome free and more nuclease sensitive in the repressed than in the transcribed locus. Mutations in genes essential for HML silencing disrupt the nucleosome array near HML-I but not in the vicinity of HML-E, which is closer to the telomere of chromosome III. At the promoter and the HO site, the structure of HMLalpha in Sir protein and histone H4 N-terminal deletion mutants is identical to that of the transcriptionally active MATalpha. The discontinuous chromatin structure of HMLalpha contrasts with the continuous array of nucleosomes found at repressed a-cell-specific genes and the recombination enhancer. Punctuation at HMLalpha may be necessary for higher-order structure or karyoskeleton interactions. The unique chromatin architecture of HMLalpha may relate to the combined requirements of transcriptional repression and recombinational competence.
遗传学研究表明，染色质结构参与了酿酒酵母中沉默交配型基因座的抑制作用。对转录沉默的HMLα和活跃的MATα进行核苷酸分辨率的染色质图谱分析表明，独特的有组织的染色质结构是HMLα沉默状态的特征。紧邻沉默子精确定位的核小体延伸至α1和α2编码区。在MATα处核酸酶敏感的HO内切核酸酶识别位点，在HMLα处受到保护。尽管两个精确定位的核小体在HMLα处包含转录起始位点，但α1和α2基因的启动子区域无核小体，且在被抑制状态下比在转录位点更易被核酸酶切割。HML沉默所必需的基因中的突变会破坏HML-I附近的核小体阵列，但不会破坏更靠近III号染色体端粒的HML-E附近的核小体阵列。在启动子和HO位点，Sir蛋白和组蛋白H4 N端缺失突变体中HMLα的结构与转录活跃的MATα的结构相同。HMLα的间断染色质结构与在被抑制的a细胞特异性基因和重组增强子处发现的连续核小体阵列形成对比。HMLα处的标点可能是高阶结构或核骨架相互作用所必需的。HMLα独特的染色质结构可能与转录抑制和重组能力的综合需求有关。
