Bell P, Mais C, McStay B, Scheer U
Department of Cell and Developmental Biology, Theodor-Boveri-Institute, University of Würzburg, Germany.
J Cell Sci. 1997 Sep;110 ( Pt 17):2053-63. doi: 10.1242/jcs.110.17.2053.
When nuclei (pronuclei) were assembled from sperm chromatin in Xenopus egg extract and examined by immunofluorescence microscopy, UBF was concentrated at a single intranuclear dot-like or more extended necklace-like structure. These UBF-foci contained rDNA as demonstrated by in situ hybridization and hence represent the chromosomal nucleolus organizing regions (NORs). Besides UBF, other components of the transcription machinery such as the TATA-box binding protein (TBP) and RNA polymerase I (pol I) as well as several nucleolar proteins could not be detected at the NORs. Immuno-depletion experiments indicated the UBF is maternally provided and taken up by the pronuclei. Essentially the same results were obtained when we examined the NORs of early Xenopus embryos up to the midblastula stage. After this stage, when transcription of the rRNA genes has begun, nucleoli developed and the NORs acquired TBP and pol I. Our results support the hypothesis that UBF is an architectural element which converts the rDNA chromatin into a transcriptionally competent form.
当在非洲爪蟾卵提取物中由精子染色质组装成细胞核（原核）并通过免疫荧光显微镜检查时，UBF集中在单个核内点状或更伸展的项链状结构上。原位杂交表明这些UBF位点含有rDNA，因此代表染色体核仁组织区（NORs）。除了UBF，在NORs处未检测到转录机制的其他成分，如TATA框结合蛋白（TBP）和RNA聚合酶I（pol I）以及几种核仁蛋白。免疫去除实验表明UBF是由母体提供并被原核吸收的。当我们检查直至囊胚中期的早期非洲爪蟾胚胎的NORs时，获得了基本相同的结果。在此阶段之后，当rRNA基因开始转录时，核仁形成，NORs获得了TBP和pol I。我们的结果支持这样的假设，即UBF是一种将rDNA染色质转化为转录活性形式的结构元件。
