Rpb5-H147R is an AT-GC transition replacing CAC(His) by CGC(Arg) at a conserved and critical position of ABC27 (Rpb5p), one of the five common and essential subunits shared by all three eukaryotic RNA polymerases. This mutation is viable at 25 degrees C, but has a lethal phenotype at 34 degrees C. A search for dosage-dependent suppressors identified five distinct clones that all bear a copy of the tRNA(His)GUG gene. Suppression was also observed with a small genomic insert bearing this tRNA gene and no other coding sequences, under conditions where there is a sevenfold increase in the cellular concentration of tRNA(His)GUG. Overexpressing tRNA(Arg)ICG, which normally decodes the suppressed CGC codon, counteracted suppression. Suppression is codon specific because it was abolished when replacing CGC by its synonymous codons CGA, CGU, or AGA, but was not detectably affected by several nucleotide substitutions modifying the surrounding sequence and is thus largely insensitive to the nucleotide context. It is proposed that overexpressing tRNA(His)GUG extends its decoding properties from CAC(His) to the noncognate CGC(Arg) codon through an illegitimate U x G pairing at the middle base of the anticodon. Accordingly, tRNA(His)GUG would compete with tRNA(Arg)ICG for chain elongation and generate a significant level of misreading errors under normal growth conditions.
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机译：Rpb5-H147R是一个AT-GC过渡区，在ABC27（Rpb5p）的保守和关键位置上被CGC（Arg）取代CAC（His），这是所有三种真核RNA聚合酶共有的五个常见和必要亚基之一。此突变在25摄氏度时是可行的，但在34摄氏度时具有致命的表型。对剂量依赖性抑制子的搜索确定了五个不同的克隆，它们都带有tRNA（His）GUG基因的拷贝。在tRNA（His）GUG细胞浓度增加7倍的条件下，还观察到带有一个带有该tRNA基因的小基因组插入片段的抑制，而没有其他编码序列。正常解码受抑制的CGC密码子的过表达tRNA（Arg）ICG可抵消抑制作用。抑制是特定于密码子的，因为当用其同义密码子CGA，CGU或AGA代替CGC时，它被取消了，但是不受修饰周围序列的几个核苷酸取代的影响，因此对核苷酸环境几乎不敏感。建议过表达的tRNA（His）GUG通过在反密码子中间碱基处的非法U x G配对将其解码特性从CAC（His）扩展到非同源CGC（Arg）密码子。因此，在正常生长条件下，tRNA（His）GUG将与tRNA（Arg）ICG竞争链延长，并产生显着水平的误读错误。
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