Foster R, Mikesell G E, Breeden L
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Mol Cell Biol. 1993 Jun;13(6):3792-801. doi: 10.1128/mcb.13.6.3792-3801.1993.
The Saccharomyces cerevisiae SWI4 gene encodes an essential transcription factor which controls gene expression at the G1/S transition of the cell cycle. SWI4 transcription itself is cell cycle regulated, and this periodicity is crucial for the normal cell cycle regulation of HO and at least two of the G1 cyclins. Since the regulation of SWI4 is required for normal cell cycle progression, we have characterized cis- and trans-acting regulators of SWI4 transcription. Deletion analysis of the SWI4 promoter has defined a 140-bp region which is absolutely required for transcription and can function as a cell cycle-regulated upstream activating sequence (UAS). The SWI4 UAS contains three potential MluI cell cycle boxes (MCBs), which are known cell cycle-regulated promoter elements. Deletion of all three MCBs in the SWI4 UAS decreases the level of SWI4 mRNA 10-fold in asynchronous cultures but does not abolish periodicity. These data suggest that MCBs are involved in SWI4 UAS activity, but at least one other periodically regulated element must be present. Since SWI6 is known to bind to MCBs and regulate their activity, the role of SWI6 in SWI4 expression was analyzed. Although the MCBs cannot account for the full cell cycle regulation of SWI4, mutations in SWI6 eliminate the normal periodicity of SWI4 transcription. This suggests that the novel cell cycle-regulated element within the SWI4 promoter is also SWI6 dependent. The constitutive transcription of SWI4 in SWI6 mutant cells occurs at an intermediate level, which indicates that SWI6 is required for the full activation and repression of SWI4 transcription through the cell cycle. It also suggests that there is another pathway which can activate SWI4 transcription in the absence of SWI6. The second activator may also target MCB elements, since SWI4 transcription drops dramatically when they are deleted.
酿酒酵母SWI4基因编码一种必需的转录因子，它在细胞周期的G1/S转换期控制基因表达。SWI4转录本身受细胞周期调控，这种周期性对于HO以及至少两种G1细胞周期蛋白的正常细胞周期调控至关重要。由于SWI4的调控是正常细胞周期进程所必需的，我们对SWI4转录的顺式和反式作用调节因子进行了表征。对SWI4启动子的缺失分析确定了一个140 bp的区域，该区域是转录绝对必需的，并且可以作为细胞周期调控的上游激活序列（UAS）发挥作用。SWI4 UAS包含三个潜在的MluI细胞周期框（MCB），它们是已知的细胞周期调控启动子元件。在SWI4 UAS中删除所有三个MCB会使异步培养物中SWI4 mRNA的水平降低10倍，但不会消除周期性。这些数据表明MCB参与了SWI4 UAS的活性，但必须存在至少一个其他周期性调节元件。由于已知SWI6与MCB结合并调节其活性，因此分析了SWI6在SWI4表达中的作用。尽管MCB不能解释SWI4的完整细胞周期调控，但SWI6中的突变消除了SWI4转录的正常周期性。这表明SWI4启动子内新的细胞周期调控元件也是SWI6依赖性的。SWI6突变细胞中SWI4的组成型转录以中等水平发生，这表明SWI6是SWI4转录在整个细胞周期中完全激活和抑制所必需的。这也表明存在另一条途径，在没有SWI6的情况下可以激活SWI4转录。第二种激活剂也可能靶向MCB元件，因为当它们被删除时SWI4转录会急剧下降。
