Larrivée Michel, Wellinger Raymund J
Department of Microbiology and Infectious Diseases, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, Quebec, J1H 5N4, Canada.
Nat Cell Biol. 2006 Jul;8(7):741-7. doi: 10.1038/ncb1429. Epub 2006 Jun 11.
Maintaining telomeric DNA at chromosome ends is essential for genome stability. In virtually all organisms the telomerase enzyme provides this function; however, telomerase-independent mechanisms also exist. These latter mechanisms rely on recombination pathways to replenish telomeric DNA and extrachromosomal DNA may also be implicated. Here, we report that in Saccharomyces cerevisiae cells, extrachromosomal circular DNA occurs for both subtypes of telomerase-independent telomere-maintenance mechanisms. This DNA consists of circular molecules of full-length subtelomeric repeat elements in type I cells, and very heterogeneously sized circles of telomeric repeat DNA in type II cells that are at least partially single stranded. Surprisingly, both type I and type II cells can adapt to a loss of the normally essential telomere-capping protein Cdc13p by inducing an alternate and reversible state of chromosome ends. Chromosome capping, therefore, is not strictly dependent on canonical capping proteins, such as Cdc13p, but can be achieved by alternate mechanisms.
维持染色体末端的端粒DNA对基因组稳定性至关重要。几乎在所有生物体中，端粒酶都发挥这一功能；然而，不依赖端粒酶的机制也存在。后一种机制依靠重组途径来补充端粒DNA，并且可能也涉及到染色体外DNA。在此，我们报道，在酿酒酵母细胞中，染色体外环状DNA出现在不依赖端粒酶的端粒维持机制的两种亚型中。这种DNA在I型细胞中由全长亚端粒重复元件的环状分子组成，在II型细胞中由大小非常不均一的端粒重复DNA环组成，这些环至少部分是单链的。令人惊讶的是，I型和II型细胞都可以通过诱导染色体末端的另一种可逆状态来适应正常情况下必不可少的端粒封端蛋白Cdc13p的缺失。因此，染色体封端并不严格依赖于典型的封端蛋白，如Cdc13p，而是可以通过其他机制实现。
