Lin J J, Zakian V A
Department of Molecular Biology, Princeton University, NJ 08544-1014, USA.
Proc Natl Acad Sci U S A. 1996 Nov 26;93(24):13760-5. doi: 10.1073/pnas.93.24.13760.
Saccharomyces telomeres consist of approximately 300 bp of C1-3A/TG1-3 DNA. Cells lacking the activity of the essential gene CDC13 display a cell cycle arrest mediated by the DNA damage sensing, RAD9 cell cycle checkpoint, presumably because they exhibit strand-specific loss of telomeric and telomere-adjacent DNA [Garvik, B., Carson, M. & Hartwell, L. (1995) Mol. Celi. Biol. 15,6128-6138]. Cdc13p expressed in Escherichia coli or overexpressed in yeast bound specifically to single-strand TG1-3 DNA. The specificity of binding displayed by Cdc13p in vitro indicates that in vivo it could bind to both the short, constitutive single-strand TG1-3 tails thought to be present at telomeres at most times in the cell cycle as well as to the long single-strand TG1-3 tails that are intermediates in telomere replication. Genes located near yeast telomeres are transcriptionally repressed, a phenomenon known as telomere position effect. Cells overexpressing a mutant form of Cdc13p had reduced telomere position effect at high temperatures. These data suggest that Cdc13p functions by binding directly to telomeric DNA, thereby limiting its accessibility to degradation and transcription as well as masking it from factors that detect damaged DNA.
酿酒酵母的端粒由大约300bp的C1-3A/TG1-3 DNA组成。缺乏必需基因CDC13活性的细胞会出现由DNA损伤感应的RAD9细胞周期检查点介导的细胞周期停滞，推测这是因为它们表现出端粒及端粒邻近DNA的链特异性缺失[Garvik, B., Carson, M. & Hartwell, L. (1995) Mol. Cell. Biol. 15,6128 - 6138]。在大肠杆菌中表达或在酵母中过表达的Cdc13p能特异性结合单链TG1-3 DNA。Cdc13p在体外表现出的结合特异性表明，在体内它既能结合细胞周期中大多数时间被认为存在于端粒处的短的、组成型的单链TG1-3尾巴，也能结合作为端粒复制中间体的长单链TG1-3尾巴。位于酵母端粒附近的基因受到转录抑制，这一现象被称为端粒位置效应。在高温下过表达Cdc13p突变形式的细胞端粒位置效应降低。这些数据表明，Cdc13p通过直接结合端粒DNA发挥作用，从而限制其被降解和转录的可及性，并使其免受检测受损DNA的因子的影响。
