Ahmad K, Henikoff S
Howard Hughes Medical Institute, and Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.
J Cell Biol. 2001 Apr 2;153(1):101-10. doi: 10.1083/jcb.153.1.101.
The properties that define centromeres in complex eukaryotes are poorly understood because the underlying DNA is normally repetitive and indistinguishable from surrounding noncentromeric sequences. However, centromeric chromatin contains variant H3-like histones that may specify centromeric regions. Nucleosomes are normally assembled during DNA replication; therefore, we examined replication and chromatin assembly at centromeres in Drosophila cells. DNA in pericentric heterochromatin replicates late in S phase, and so centromeres are also thought to replicate late. In contrast to expectation, we show that centromeres replicate as isolated domains early in S phase. These domains do not appear to assemble conventional H3-containing nucleosomes, and deposition of the Cid centromeric H3-like variant proceeds by a replication-independent pathway. We suggest that late-replicating pericentric heterochromatin helps to maintain embedded centromeres by blocking conventional nucleosome assembly early in S phase, thereby allowing the deposition of centromeric histones.
在复杂真核生物中，定义着丝粒的特性仍知之甚少，因为其 underlying DNA 通常具有重复性，且与周围的非着丝粒序列难以区分。然而，着丝粒染色质包含变体 H3 样组蛋白，这些组蛋白可能会指定着丝粒区域。核小体通常在 DNA 复制过程中组装；因此，我们研究了果蝇细胞中着丝粒的复制和染色质组装。着丝粒周围异染色质中的 DNA 在 S 期后期复制，所以着丝粒也被认为是在后期复制。与预期相反，我们发现着丝粒在 S 期早期作为孤立的结构域进行复制。这些结构域似乎不会组装传统的含 H3 核小体，并且 Cid 着丝粒 H3 样变体的沉积通过不依赖复制的途径进行。我们认为，后期复制的着丝粒周围异染色质通过在 S 期早期阻止传统核小体的组装，从而有助于维持嵌入其中的着丝粒，进而允许着丝粒组蛋白的沉积。
