A Sequence Element Downstream of the Yeast HTB1 Gene Contributes to mRNA 3′ Processing and Cell Cycle Regulation

Histone mRNAs accumulate in the S phase and are rapidly degraded as cells progress into the G(2) phase of the cell cycle. In Saccharomyces cerevisiae, fusion of the 3′ untranslated region and downstream sequences of the yeast histone gene HTB1 to a neomycin phosphotransferase open reading frame is sufficient to confer cell cycle regulation on the resulting chimera gene (neo-HTB1). We have identified a sequence element, designated the distal downstream element (DDE), that influences both the 3′-end cleavage site selection and the cell cycle regulation of the neo-HTB1 mRNA. Mutations in the DDE, which is located approximately 110 nucleotides downstream of the HTB1 gene, lead to a delay in the accumulation of the neo-HTB1 mRNA in the S phase and a lack of mRNA turnover in the G(2) phase. The DDE is transcribed as part of the primary transcript and binds a protein factor(s). Maximum binding is observed in the S phase of the cell cycle, and mutations that affect the turnover of the HTB1 mRNA alter the binding activity. While located in the same general region, mutations that affect 3′-end cleavage site selection act independently from those that alter the cell cycle regulation.组蛋白mRNA（Histone mRNAs）会在细胞周期的S期（S phase）积累，并在细胞进入G2期（G2 phase）后被快速降解。在酿酒酵母（Saccharomyces cerevisiae）中，将酵母组蛋白基因HTB1的3'非翻译区（3′ untranslated region）及其下游序列与新霉素磷酸转移酶开放阅读框（neomycin phosphotransferase open reading frame）融合，即可使所得嵌合基因（neo-HTB1）获得细胞周期调控特性。本研究已鉴定出一种被命名为远端下游元件（distal downstream element，DDE）的序列元件，其可同时影响neo-HTB1 mRNA的3'末端切割位点选择与细胞周期调控活性。该远端下游元件位于HTB1基因下游约110个核苷酸处，其突变会导致neo-HTB1 mRNA在S期的积累延迟，并使G2期丧失mRNA降解周转能力。该远端下游元件作为初级转录本（primary transcript）的一部分被转录，并可结合一种或多种蛋白质因子（protein factor）。其结合活性在细胞周期的S期达到峰值，且影响HTB1 mRNA降解周转的突变会改变该结合活性。尽管影响3'末端切割位点选择与细胞周期调控的突变处于同一大致区域，但二者的作用机制彼此独立。