A growth rate-limiting process in the last growth phase of the yeast life cycle involves RPB4, a subunit of RNA polymerase II.

Saccharomyces cerevisiae cells, grown on a fermentable carbon source, display two growth phases before they enter the stationary phase: a rapid phase (log phase) followed by a slow phase. It was previously shown that a subunit of the yeast RNA polymerase II, RPB4, positively affects the activity of the enzyme in post-log phases but has little or no effect on its activity in log phase. Here, I show that RPB4 level limits the growth rate during the slow growth phase. Thus, a small increase in RPB4 protein level, in cells carrying multiple copies of the RPB4 gene, results in an almost twofold increase in the growth rate during this phase. Furthermore, RPB4 expression is differentially regulated in the two growth phases. During the slow growth phase, a posttranscriptional process which controls the RPB4 level and thus can control growth rate becomes active. These results reveal a complex growth control mechanism, in which the transcriptional apparatus is probably a limiting element, operating in the last stages of the yeast growth.