Maric C, Hyrien O
Laboratoire de Génétique Moléculaire, Ecole Normale Supérieure, 46 Rue d'Ulm, F-75230 Paris Cedex 05, France.
Chromosoma. 1998 Jun;107(3):155-65. doi: 10.1007/s004120050292.
We have investigated the possible relationship between replicons and chromatin loops during Xenopus development. In early embryos, replication of the ribosomal RNA genes (rDNA) can initiate at apparently any sequence. Nevertheless, the need for a regular spacing of replication origins suggests that some periodic chromatin folding might dictate which sites are actually used for initiation. After the midblastula transition, replication initiation is restricted to the rDNA intergenic spacers. A remodeling of chromatin folding could account for this change in origin usage. Here, it is reported that nuclear matrix anchorage of the Xenopus rDNA occurs at multiple, apparently random sequences, throughout embryonic development as well as in adult cells. In vitro matrix rebinding assays confirmed the lack of specific anchoring sequences in the rDNA, before as well as after specific replication origins are established. Thus, no change in loop attachment sites could explain the change in origin usage at this locus. Nonspecific loop anchorage was a special feature of the rDNA locus, since the same nuclear matrices were able selectively to bind the scaffold attachment region (SAR) of the Drosophila histone gene cluster in vitro. Blastula and gastrula nuclear matrices bound a higher amount of SAR sequences than matrices from later stages or adult cells. This developmental change in SAR binding might explain the increase in size of the bulk of genomic DNA loops that occurs after the gastrula stage. However, no change in chromatin loop organization that could explain the midblastula stage transition from small to large replicons was observed.
我们研究了非洲爪蟾发育过程中复制子与染色质环之间可能存在的关系。在早期胚胎中，核糖体RNA基因（rDNA）的复制似乎可以在任何序列起始。然而，复制起点需要有规律的间隔，这表明某些周期性的染色质折叠可能决定了哪些位点实际上被用于起始复制。在囊胚中期转变之后，复制起始被限制在rDNA基因间隔区。染色质折叠的重塑可以解释这种起始点使用的变化。在此报道，非洲爪蟾rDNA的核基质锚定在整个胚胎发育过程以及成年细胞中都发生在多个明显随机的序列处。体外基质再结合试验证实，在特定复制起点建立之前和之后，rDNA中都缺乏特定的锚定序列。因此，环附着位点的变化无法解释该位点起始点使用的变化。非特异性环锚定是rDNA位点的一个特殊特征，因为相同的核基质能够在体外选择性地结合果蝇组蛋白基因簇的支架附着区域（SAR）。囊胚期和原肠胚期的核基质比后期阶段或成年细胞的核基质结合更多的SAR序列。这种SAR结合的发育变化可能解释了原肠胚期后基因组DNA环总体大小的增加。然而，未观察到能解释囊胚中期从小复制子向大复制子转变的染色质环组织变化。
