In many species, centromere identity is specified epigenetically by special nucleosomes containing a centromere-specific histone H3 variant, designated as CENP-A in humans and CID in Drosophila melanogaster. After partitioning of centromere-specific nucleosomes onto newly replicated sister centromeres, loading of additional CENP-A/CID into centromeric chromatin is required for centromere maintenance in proliferating cells. Analyses with cultured cells have indicated that transcription of centromeric DNA by RNA polymerase II is required for deposition of new CID into centromere chromatin. However, a dependence of centromeric CID loading on transcription is difficult to reconcile with the notion that the initial embryonic stages appear to proceed in the absence of transcription in Drosophila, as also in many other animal species. To address the role of RNA polymerase II–mediated transcription for CID loading in early Drosophila embryos, we have quantified the effects of alpha-amanitin and triptolide on centromeric CID-EGFP levels. Our analyses demonstrate that microinjection of these two potent inhibitors of RNA polymerase II–mediated transcription has at most a marginal effect on centromeric CID deposition during progression through the early embryonic cleavage cycles. Thus, we conclude that at least during early Drosophila embryogenesis, incorporation of CID into centromeres does not depend on RNA polymerase II–mediated transcription.

在许多物种中，着丝粒同一性是由特殊核小体表观遗传指定的，这些核小体含有着丝粒特异性组蛋白 H3 变体，在人类中称为 CENP-A，在黑腹果蝇中称为CID。在将着丝粒特异性核小体分配到新复制的姐妹着丝粒上后，需要将额外的 CENP-A/CID 加载到着丝粒染色质中，以维持增殖细胞中的着丝粒。对培养细胞的分析表明，新的 CID 沉积到着丝粒染色质中需要 RNA 聚合酶 II 转录着丝粒 DNA。然而，着丝粒 CID 加载对转录的依赖性很难与果蝇中初始胚胎阶段似乎在没有转录的情况下进行的观点相一致，与许多其他动物物种一样。为了解决 RNA 聚合酶 II 介导的转录在早期果蝇胚胎中加载 CID 的作用，我们量化了 alpha-amanitin 和雷公藤内酯对着丝粒 CID-EGFP 水平的影响。我们的分析表明，显微注射这两种有效的 RNA 聚合酶 II 介导的转录抑制剂在早期胚胎卵裂周期的进展过程中至多对着丝粒 CID 沉积具有边际影响。因此，我们得出结论，至少在早期果蝇胚胎发生期间，CID 并入着丝粒不依赖于 RNA 聚合酶 II 介导的转录。