Breviario D, Hinnebusch A G, Dhar R
Laboratory of Molecular Virology, National Cancer Institute, Bethesda, MD 20892.
EMBO J. 1988 Jun;7(6):1805-13. doi: 10.1002/j.1460-2075.1988.tb03012.x.
Expression of the RAS1 and RAS2 genes of Saccharomyces cerevisiae has been examined at the transcriptional and translational levels. When dextrose is the carbon source, the steady-state amount of RAS1 mRNA and the rate of RAS1 protein synthesis are reduced in parallel as cells approach the mid-exponential phase of growth. RAS1 mRNA levels and protein synthesis are very low at all stages of growth when ethanol rather than dextrose is provided as the sole carbon source. The rate of RAS2 protein synthesis is regulated differently. In cells cultured on dextrose, it is lowest in the early exponential phase, increases approximately 10-fold and remains nearly constant as cells approach stationary phase. By contrast, RAS2 mRNA is found at uniformly high levels at all phases of exponential growth, suggesting that the translational efficiency of RAS2 mRNA is repressed during the early exponential phase. This repression is not observed when ethanol is the sole carbon source. Nutrient starvation, resulting in G1 arrest and sporulation in diploids, leads to greatly decreased amounts of RAS2 mRNA, accomplished in part by selective repression of RAS2 transcripts with particular 5' ends. However, this reduction in RAS2 mRNA levels has little effect on the rate of RAS2 protein synthesis, suggesting that the translational efficiency of RAS2 mRNA is stimulated by nutrient starvation. The combination of transcriptional and translational controls which regulate yeast RAS gene expression seems to ensure that one or the other RAS proteins will be produced over a wide range of physiological states.
已在转录和翻译水平上研究了酿酒酵母RAS1和RAS2基因的表达。当葡萄糖作为碳源时，随着细胞接近生长的指数中期，RAS1 mRNA的稳态量和RAS1蛋白合成速率会平行降低。当以乙醇而非葡萄糖作为唯一碳源时，RAS1 mRNA水平和蛋白合成在生长的所有阶段都非常低。RAS2蛋白合成速率的调节方式不同。在以葡萄糖培养的细胞中，它在指数早期阶段最低，随着细胞接近稳定期增加约10倍并几乎保持恒定。相比之下，在指数生长的所有阶段都发现RAS2 mRNA处于均匀的高水平，这表明RAS2 mRNA的翻译效率在指数早期阶段受到抑制。当乙醇是唯一碳源时，未观察到这种抑制作用。营养饥饿导致二倍体的G1期停滞和孢子形成，导致RAS2 mRNA的量大大减少，部分原因是对具有特定5'末端的RAS2转录本的选择性抑制。然而，RAS2 mRNA水平的这种降低对RAS2蛋白合成速率几乎没有影响，这表明营养饥饿会刺激RAS2 mRNA的翻译效率。调节酵母RAS基因表达的转录和翻译控制的组合似乎确保在广泛的生理状态下会产生一种或另一种RAS蛋白。
