Expression of the rasl and ras2 genes of Saccharomyces cerevisiae has been examined at the transcriptional and translational levels. In cells grown with glucose as carbon source, rasl mRNA and rasl protein synthesis were detected only in the early exponential phase of growth. By contrast, ras2 protein synthesis was low in the early exponential phase, increased 10-fold and remained nearly constant into the stationary phase. The ras2 mRNA level was high and nearly constant until late in the exponential phase and decreased considerably as cells entered the stationary phase. Taken together, these data suggest that translational control is important in regulating ras2 gene expression in cells grown on glucose. Nutrient starvation, leading to G1-arrest and sporulation in diploids, had little effect on the rate of ras2 protein synthesis, but lead to decreased amounts of ras2 mRNA. This decrease was accomplished in part by selective repression of ras2 transcripts with particular 5' ends. Our data also suggest that nutrient starvation is another condition in which translational control is prominent in regulation of ras2 expression. The fact that a large decrease in the amount of ras2 mRNA occurs in the stationary phase and starvation conditions but is associated with little effect on ras2 protein synthesis, suggests that ras2 transcriptional control in these conditions is designed primarily to offset changes in translational efficiency.
