Windle J, Sollner-Webb B
Mol Cell Biol. 1986 Apr;6(4):1228-34. doi: 10.1128/mcb.6.4.1228-1234.1986.
The DNA sequences involved in promoting transcription of the Xenopus laevis rRNA genes were determined by microinjecting a series of deletion mutants into oocyte nuclei. A very small promoter region is sufficient to direct efficient transcription when templates are microinjected at high rDNA concentration, since 5'delta- 9 and 3'delta +6 templates are fully active. However, as the concentration of injected template is decreased, an increasing requirement for upstream domains, extending to nucleotide approximately -170, is observed. The major downstream border of the required region does not change. This apparently expanding 5' promoter border results from the fact that, as the rDNA concentration is decreased, transcription from templates lacking the upstream promoter domain falls off much more sharply than does transcription from a complete promoter. In fact, the deleted promoters are virtually inactive below a threshold rDNA concentration. It is indeed the rDNA concentration that is important, for coinjected vector DNA does not increase the level of transcription obtained from low concentrations of the 5' deletions. From these data we conclude that polymerase I transcription factors can recognize and initiate transcription from a small core promoter domain, but that sequences extending upstream to nucleotide approximately -170 increase the efficiency of initiation. A model is presented that could account for these results.
通过将一系列缺失突变体显微注射到卵母细胞核中，确定了非洲爪蟾rRNA基因转录起始相关的DNA序列。当以高rDNA浓度显微注射模板时，一个非常小的启动子区域就足以指导高效转录，因为5'端缺失9个核苷酸和3'端缺失6个核苷酸的模板仍具有完全活性。然而，随着注射模板浓度的降低，对上游区域的需求增加，该区域一直延伸到大约-170位核苷酸处，而所需区域的主要下游边界没有变化。这种明显不断扩展的5'端启动子边界是由于这样一个事实：随着rDNA浓度降低，缺乏上游启动子区域的模板转录比完整启动子的转录下降得更为急剧。实际上，在低于阈值rDNA浓度时，缺失的启动子几乎没有活性。重要的确实是rDNA浓度，因为共注射的载体DNA并不能提高从低浓度5'端缺失模板获得的转录水平。从这些数据我们得出结论，聚合酶I转录因子能够识别一个小的核心启动子区域并从该区域起始转录，但延伸到大约-170位核苷酸的上游序列会提高起始效率。本文提出了一个可以解释这些结果的模型。
