Multiple cellular processes affected by the absence of the Rpb4 subunit ofRNA polymerase II contribute to the deficiency in the stress response of the yeast rpb4 Delta mutant

We previously described the isolation of yeast mutants (sex mutants) that s ecrete reduced amounts of mature alpha -factor when it is synthesized as pa rt of a fusion with prosomatostatin. In the present study we show that the sex3-1 mutant displays pleiotropic phenotypes. These include an abnormal mo rphology, an osmoremediable caffeine sensitivity, reduced secretion of matu re alpha -factor, a weakened cell wall and a marked deficiency in halotoler ance. Cloning of the SEX3 gene revealed that it is identical to the RPB4 ge ne. This gene encodes the fourth largest subunit of yeast RNA polymerase II , which has been postulated to play a major role in the response to stress. We show that transcriptional activation in response to either a cell wall stress or to growth in the presence of elevated salt concentrations is mini mally affected by the loss of RPB4 function. However, whereas the levels of several mRNAs are similarly reduced (by about 30%) in rpb4 mutants grown i n rich medium at moderate temperature, some transcripts, in particular ZDS1 , are more abundant. An increase dosage of ZDS1, or of genes involved in ce ll wall assembly and in secretion (RHO1 and SRO77, respectively), partially suppresses the sensitivity of rpb4 Delta cells to high temperature, heat s hock and stationary phase. Collectively, our results indicate that the loss of Rpb4p perturbs several cellular functions that contribute to the inappr opriate stress response of rpb4 Delta yeast. We therefore conclude that thi s RNA polymerase II subunit is not specifically involved in the stress resp onse.