Department of Biomolecular Chemistry, School of Medicine and Public Health, University of Wisconsin, Madison, Wisconsin, United States of America.
PLoS Genet. 2009 Dec;5(12):e1000755. doi: 10.1371/journal.pgen.1000755. Epub 2009 Dec 4.
The origin recognition complex (ORC) marks chromosomal sites as replication origins and is essential for replication initiation. In yeast, ORC also binds to DNA elements called silencers, where its primary function is to recruit silent information regulator (SIR) proteins to establish transcriptional silencing. Indeed, silencers function poorly as chromosomal origins. Several genetic, molecular, and biochemical studies of HMR-E have led to a model proposing that when ORC becomes limiting in the cell (such as in the orc2-1 mutant) only sites that bind ORC tightly (such as HMR-E) remain fully occupied by ORC, while lower affinity sites, including many origins, lose ORC occupancy. Since HMR-E possessed a unique non-replication function, we reasoned that other tight sites might reveal novel functions for ORC on chromosomes. Therefore, we comprehensively determined ORC "affinity" genome-wide by performing an ORC ChIP-on-chip in ORC2 and orc2-1 strains. Here we describe a novel group of orc2-1-resistant ORC-interacting chromosomal sites (ORF-ORC sites) that did not function as replication origins or silencers. Instead, ORF-ORC sites were comprised of protein-coding regions of highly transcribed metabolic genes. In contrast to the ORC-silencer paradigm, transcriptional activation promoted ORC association with these genes. Remarkably, ORF-ORC genes were enriched in proximity to origins of replication and, in several instances, were transcriptionally regulated by these origins. Taken together, these results suggest a surprising connection among ORC, replication origins, and cellular metabolism.
复制起始复合物(ORC)标记染色体位点作为复制起点,对复制起始至关重要。在酵母中,ORC 还与称为沉默子的 DNA 元件结合,其主要功能是招募沉默信息调节因子(SIR)蛋白以建立转录沉默。事实上,沉默子作为染色体起点的功能很差。对 HMR-E 的多项遗传、分子和生化研究导致提出了一个模型,即当 ORC 在细胞中变得有限(例如在 orc2-1 突变体中)时,只有紧密结合 ORC 的位点(如 HMR-E)仍然完全被 ORC 占据,而低亲和力位点,包括许多起点,会失去 ORC 占有率。由于 HMR-E 具有独特的非复制功能,我们推断其他紧密结合的位点可能会揭示 ORC 在染色体上的新功能。因此,我们通过在 ORC2 和 orc2-1 菌株中进行 ORC ChIP-on-chip 实验,全面确定了 ORC 的“亲和力”全基因组图谱。在这里,我们描述了一组新的 orc2-1 抗性 ORC 相互作用的染色体位点(ORF-ORC 位点),它们不能作为复制起点或沉默子发挥作用。相反,ORF-ORC 位点由高度转录的代谢基因的编码区组成。与 ORC-沉默子范例相反,转录激活促进了这些基因与 ORC 的结合。值得注意的是,ORF-ORC 基因在复制起点附近富集,并且在某些情况下,这些基因受到这些起点的转录调控。总之,这些结果表明 ORC、复制起点和细胞代谢之间存在令人惊讶的联系。