Parker R, Simmons T, Shuster E O, Siliciano P G, Guthrie C
Department of Biochemistry and Biophysics, University of California, San Francisco 94143.
Mol Cell Biol. 1988 Aug;8(8):3150-9. doi: 10.1128/mcb.8.8.3150-3159.1988.
Saccharomyces cerevisiae contains at least 24 distinct small nuclear RNAs (snRNAs), several of which are known to be essential for viability and to participate in the splicing of pre-mRNAs; the RNAs in this subset contain binding sites for the Sm antigen, a hallmark of metazoan snRNAs involved in mRNA processing. In contrast, we showed previously that the single-copy genes for three other snRNAs (snR3, snR4, and snR10) are not required for viability, although cells lacking snR10 are growth impaired at low temperature. None of these RNAs associates with the Sm antigen. To assess this apparent correlation, we cloned and sequenced the genes encoding three additional non-Sm snRNAs. Comparison of these genes with nine additional yeast snRNA genes revealed a highly conserved TATA box located 92 +/- 8 nucleotides 5' of the transcriptional start site. By using the technique of gene replacement with null alleles, each of these three single copy genes was shown to be completely dispensable. We constructed multiple mutants to test the hypothesis that, individually, each of these snRNAs is nonessential because the snRNAs play functionally overlapping roles. A mutant lacking five snRNAs (snR3, snR4, snR5, snR8, snR9) was indistinguishable from the wild type, and growth of the sextuple mutant was no more impaired than that in strains lacking only snR10. This widespread dispensability of snRNAs was completely unexpected and forces us to reconsider the possible roles of these ubiquitous RNAs.
酿酒酵母至少含有24种不同的小核RNA（snRNA），其中几种已知对细胞存活至关重要，并参与前体mRNA的剪接；该亚组中的RNA含有Sm抗原的结合位点，Sm抗原是参与mRNA加工的后生动物snRNA的一个标志。相比之下，我们之前表明，另外三种snRNA（snR3、snR4和snR10）的单拷贝基因对细胞存活不是必需的，尽管缺乏snR10的细胞在低温下生长受损。这些RNA都不与Sm抗原结合。为了评估这种明显的相关性，我们克隆并测序了编码另外三种非Sm snRNA的基因。将这些基因与另外九个酵母snRNA基因进行比较，发现在转录起始位点5'端92±8个核苷酸处有一个高度保守的TATA框。通过使用无义等位基因进行基因替换的技术，表明这三个单拷贝基因中的每一个都是完全可有可无的。我们构建了多个突变体来测试这样一个假设，即这些snRNA中的每一个单独来说都是非必需的，因为这些snRNA发挥着功能重叠的作用。一个缺乏五种snRNA（snR3、snR4、snR5、snR8、snR9）的突变体与野生型没有区别，六重突变体的生长受损程度并不比仅缺乏snR10的菌株更严重。snRNA这种广泛的可有可无性完全出乎意料，迫使我们重新考虑这些普遍存在的RNA可能发挥的作用。
