Zaret K S, Sherman F
J Mol Biol. 1984 Jul 25;177(1):107-35. doi: 10.1016/0022-2836(84)90060-3.
The cyc1-512 mutant of the yeast Saccharomyces cerevisiae contains a 38 base-pair deletion in the 3' non-coding region of the CYC1 gene, which encodes iso-1-cytochrome c. The deletion affects the CYC1 terminator, causing CYC1 mRNAs to be much longer and more unstable than normal. Previous genetic analysis of revertants of the cyc1-512 mutant indicated that the defect could be completely or partially restored by three classes of genetic events: chromosomal rearrangements; local genetic changes near the original cyc1-512 mutation; and suppressors at unlinked loci. We show that all the revertants with chromosomal rearrangements have breakpoints 3' to the CYC1 locus, resulting in the formation of CYC1 mRNA with new 3' non-coding regions and new 3' mRNA termini. One spontaneous cyc1-512 revertant has a 3' insertion that resembles a repetitive, transposable yeast sequence (Ty1); CYC1 transcripts end just within the bounds of this element. This study reveals that the different 3' non-coding sequences, which arose by chromosomal rearrangements, increase the stability of CYC1 mRNA and have varying effects upon the mRNA translational efficiency. Many of the cyc1-512 revertants contain only local genetic changes that create stronger terminators from the weak terminators observed in the cyc1-512 mutant. Several types of terminators in these revertants have been identified; some cause discrete termination over a relatively small region, while others cause heterogeneous termination over a 200 base-pair region. The DNA sequence changes for two cyc1-512 revertants occur in a region with homology to a consensus sequence for transcription termination in yeast that was proposed by Zaret & Sherman (1982). Two classes of extragenic suppressors of the cyc1-512 mutation have been identified. One class of the suppressors appears specifically to enhance termination at weak terminator sites, while the other class of suppressors appears to increase the stability of aberrantly long CYC1 mRNA. The results from this study support our previous suggestion (Zaret & Sherman, 1982) that, in contrast to the usual situation in higher eukaryotes, transcription termination and polyadenylation may be coupled processes in yeast.
酿酒酵母的cyc1 - 512突变体在编码同工型1 - 细胞色素c的CYC1基因的3'非编码区含有一个38个碱基对的缺失。该缺失影响CYC1终止子，导致CYC1 mRNA比正常情况长得多且更不稳定。先前对cyc1 - 512突变体回复子的遗传分析表明，三类遗传事件可完全或部分恢复该缺陷：染色体重排；原始cyc1 - 512突变附近的局部遗传变化；以及不连锁位点的抑制子。我们发现，所有具有染色体重排的回复子在CYC1基因座3'端都有断点，导致形成具有新的3'非编码区和新的3' mRNA末端的CYC1 mRNA。一个自发的cyc1 - 512回复子有一个3'插入，类似于一个重复的、可转座的酵母序列（Ty1）；CYC1转录本正好在这个元件的范围内结束。这项研究表明，由染色体重排产生的不同3'非编码序列增加了CYC1 mRNA的稳定性，并对mRNA翻译效率有不同影响。许多cyc1 - 512回复子只含有局部遗传变化，这些变化从cyc1 - 512突变体中观察到的弱终止子产生更强的终止子。已在这些回复子中鉴定出几种类型的终止子；一些在相对较小的区域导致离散终止，而另一些在200个碱基对的区域导致异质终止。两个cyc1 - 512回复子的DNA序列变化发生在与Zaret和Sherman（1982）提出的酵母转录终止共有序列具有同源性的区域。已鉴定出cyc1 - 512突变的两类基因外抑制子。一类抑制子似乎专门增强弱终止子位点的终止，而另一类抑制子似乎增加异常长的CYC1 mRNA的稳定性。这项研究的结果支持了我们先前的建议（Zaret和Sherman，1982），即与高等真核生物的通常情况相反，转录终止和多聚腺苷酸化在酵母中可能是偶联的过程。
