Bakken A, Morgan G, Sollner-Webb B, Roan J, Busby S, Reeder R H
Proc Natl Acad Sci U S A. 1982 Jan;79(1):56-60. doi: 10.1073/pnas.79.1.56.
We have injected cloned derivatives of Xenopus laevis ribosomal genes into X. laevis oocyte nuclei and examined the resulting transcription complexes in the electron microscope. From this work we conclude that the promoter lies somewhere within a region between -320 nucleotides upstream and +113 nucleotides downstream from the site of transcription initiation. This assignment agrees with inferences based on sequence conservation. It further suggests that the duplicated initiation region sequences located further out in the spacer ("Bam islands") are not required for the normal high densities of RNA polymerase loading seen on ribosomal genes. Concerning termination, the cluster of four Ts that forms part of the HindIII restriction site at the 3' end of the gene appears to be part of the normal termination signal. Termination still occurs when only three Ts are present, but reduction to two Ts damages termination. Because clusters of three Ts appear at several sites within the gene, it is likely that sequences adjacent to the T cluster also are required for normal termination. In addition, we present evidence for a fail-safe termination site just upstream from the site of transcription initiation.
我们已将非洲爪蟾核糖体基因的克隆衍生物注入非洲爪蟾卵母细胞核，并在电子显微镜下检查了由此产生的转录复合体。通过这项工作我们得出结论，启动子位于转录起始位点上游 -320 个核苷酸至下游 +113 个核苷酸之间的某个区域内。这一定位与基于序列保守性的推断相符。它还表明，位于间隔区更远位置的重复起始区域序列（“Bam 岛”）对于核糖体基因上正常的高浓度 RNA 聚合酶装载并非必需。关于终止，在基因 3' 端形成 HindIII 限制性位点一部分的四个 T 簇似乎是正常终止信号的一部分。当只有三个 T 时终止仍会发生，但减少到两个 T 会损害终止。由于三个 T 的簇出现在基因内的几个位点，与 T 簇相邻的序列可能对于正常终止也是必需的。此外，我们提供了转录起始位点上游存在一个故障安全终止位点的证据。
