Clinical Sciences Research Institute, Gibbet Hill Campus, University of Warwick, Coventry, United Kingdom.
PLoS Genet. 2011 Mar;7(3):e1001328. doi: 10.1371/journal.pgen.1001328. Epub 2011 Mar 10.
Asymmetrical segregation of differentiated sister chromatids is thought to be important for cellular differentiation in higher eukaryotes. Similarly, in fission yeast, cellular differentiation involves the asymmetrical segregation of a chromosomal imprint. This imprint has been shown to consist of two ribonucleotides that are incorporated into the DNA during lagging-strand synthesis in response to a replication pause, but the underlying mechanism remains unknown. Here we present key novel discoveries important for unravelling this process. Our data show that cis-acting sequences within the mat1 cassette mediate pausing of replication forks at the proximity of the imprinting site, and the results suggest that this pause dictates specific priming at the position of imprinting in a sequence-independent manner. Also, we identify a novel type of cis-acting spacer region important for the imprinting process that affects where subsequent primers are put down after the replication fork is released from the pause. Thus, our data suggest that the imprint is formed by ligation of a not-fully-processed Okazaki fragment to the subsequent fragment. The presented work addresses how differentiated sister chromatids are established during DNA replication through the involvement of replication barriers.
分化姐妹染色单体的不对称分离被认为对高等真核生物的细胞分化很重要。同样，在裂殖酵母中，细胞分化涉及到染色体印记的不对称分离。已经表明，这种印记由两个核苷酸组成，它们在滞后链合成过程中响应复制暂停而被掺入 DNA 中，但潜在的机制仍然未知。在这里，我们提出了对揭示这一过程至关重要的新发现。我们的数据表明，mat1 盒内的顺式作用序列介导了复制叉在印记位点附近的暂停，结果表明，这种暂停以序列非依赖性的方式决定了在印记位置的特异性引发。此外，我们还鉴定了一种新型的顺式作用间隔区，它对印记过程很重要，影响复制叉从暂停中释放后后续引物的放置位置。因此，我们的数据表明，印记是通过将一个未完全加工的冈崎片段连接到随后的片段而形成的。本研究通过涉及复制障碍，解决了在 DNA 复制过程中如何建立分化的姐妹染色单体的问题。
