YEAST RNA VIROLOGY

我们已经描述了两种双链RNA病毒(L-A和L-BC)和两种单链RNA 酵母中的复制子(20S RNA和23S RNA)。 MdsRNA是L-A的一个卫星，编码杀手毒素。我们发现 7个染色体基因，SKI1，2，3，4，6，7和8，根据它们的能力 防止这些复制子对酵母细胞致病。 这四个RNA复制子都产生无帽的mRNAs，并且缺少3‘聚(A) 结构。SKI1是一种外切核糖核酸酶，专用于无帽的mRNAs， 虽然我们发现Ski2、SKI3和SKI8基因产物阻止了 非聚(A)mRNAs的翻译。我们发现了20个基因突变 在1960年代，导致M dsRNA丢失的染色体基因是缺陷的 核糖体亚基。这些突变被SKI突变抑制 而没有恢复60S亚基缺陷。我们建议说 SKI2、SKI3和SKI8通过一种效应阻断非Poly(A)mRNA的翻译 影响60S亚基与核糖体相互作用的核糖体生物发生 信使核糖核酸的3‘多聚(A)结构。我们现在已经克隆了SKI6和 SKI7基因。Ski6P与RNase PH同源，RNase PH是一种去除 来自磷酸分解反应中tRNA前体的3‘核苷酸(如 多核苷酸磷酸化酶)。Ski6-2突变体对 潮霉素，Ski6p参与翻译的一个迹象。这些 SKI6-2菌株翻译非聚(A)mRNAs比同基因野生型高10倍 键入单元格。它们还积累了一个38S粒子，其中部分 降解的25S rRNA，为60S亚基的改变提供了直接证据 作为非Poly(A)mRNAs去抑制翻译的原因。Ski7p 与翻译因子EF1α高度同源。Ski7突变体 去抑制非聚(A)mRNAs的翻译和过度表达 Ski7p的缺失导致M dsRNA的丢失。

We have described two dsRNA viruses (L-A and L-BC) and two ssRNA replicons (20S RNA and 23S RNA) in the yeast Saccharomyces cerevisiae. M dsRNA is a satellite of L-A encoding the killer toxin. We discovered 7 chromosomal genes, SKI1, 2, 3, 4, 6, 7, and 8, by their ability to prevent these replicons from causing pathogenicity to yeast cells. These four RNA replicons all make uncapped mRNAs and lack a 3' poly(A) structure. SKI1 is an exoribonuclease specific for uncapped mRNAs, while we showed that the SKI2, SKI3 and SKI8 gene products block the translation of non-poly(A) mRNAs. We showed that mutations in 20 chromosomal genes resulting in loss of M dsRNA are deficient in 60S ribosomal subunits. These mutations are suppressed by ski mutations without restoration of the 60S subunit deficiency. We proposed that SKI2, SKI3 and SKI8 block translation of non-poly(A) mRNA by an effect on ribosome biogenesis affecting the interaction of 60S subunits with the 3' poly(A) structure of the mRNA. We have now cloned the SKI6 and SKI7 genes. Ski6p has homology with RNase PH, an enzyme that removes 3' nucleotides from tRNA precursors in a phosphorolytic reaction (like polynucleotide phosphorylase). Ski6-2 mutants are hypersensitive to hygromycin, an indication of involvement of Ski6p in translation. These ski6-2 strains translate non-poly(A) mRNAs 10x better than isogenic wild type cells. They also accumulate a 38S particle with a partially degraded 25S rRNA, providing direct evidence for altered 60S subunits as the cause of the derepressed translation of non-poly(A) mRNAs. Ski7p is highly homologous to translation factor EF1alpha. The ski7 mutants are derepressed for translation of non-poly(A) mRNAs and overexpression of Ski7p results in loss of M dsRNA.