Prieto J-L, McStay B
Biomedical Research Centre, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, UK.
Biochem Soc Trans. 2005 Dec;33(Pt 6):1441-3. doi: 10.1042/BST0331441.
The nucleolus is the site of rRNA transcription, pre-rRNA processing and ribosome subunit assembly. The nucleolus assembles around clusters of ribosomal gene repeats during late telophase, persists throughout interphase and then disassembles as cells enter mitosis. The initial step in nucleolar formation is ribosomal gene transcription, which is mediated by Pol I (RNA polymerase I) and its associated transcription factors: UBF (upstream-binding factor), SL1 (selectivity factor) and TIF-IA (transcription initiation factor IA)/Rrn3. Ribosomal gene clusters, termed NORs (nucleolar organizer regions), are found on each of the five human acrocentric chromosomes. Though transcription is repressed during metaphase, NORs that were active in the previous interphase form prominent cytogenetic features, namely secondary constrictions. The main defining characteristic of these constrictions is under-condensation in comparison with the rest of the chromosome. Extensive binding of UBF over the ribosomal gene repeat is responsible for the formation of this chromosomal feature. During interphase, the majority of the Pol I transcription machinery, though present in nucleoli, is not actively engaged in transcription. Interaction with UBF bound across the gene repeat provides an explanation for how this non-engaged Pol I machinery is sequestered by nucleoli.
核仁是核糖体RNA转录、前体核糖体RNA加工和核糖体亚基组装的场所。在末期晚期，核仁围绕核糖体基因重复簇组装，在整个间期持续存在，然后在细胞进入有丝分裂时解体。核仁形成的第一步是核糖体基因转录，这由RNA聚合酶I（Pol I）及其相关转录因子介导：上游结合因子（UBF）、选择性因子（SL1）和转录起始因子IA（TIF-IA）/Rrn3。核糖体基因簇，称为核仁组织区（NORs），存在于人类五条近端着丝粒染色体的每一条上。虽然在中期转录受到抑制，但在前一个间期活跃的NORs形成了突出的细胞遗传学特征，即次级缢痕。这些缢痕的主要定义特征是与染色体的其余部分相比凝聚不足。UBF在核糖体基因重复序列上的广泛结合导致了这种染色体特征的形成。在间期，大多数Pol I转录机制虽然存在于核仁中，但并不积极参与转录。与跨越基因重复序列结合的UBF相互作用，解释了这种未参与转录的Pol I机制是如何被核仁隔离的。
