Mutation that fasten growth of S. cerevisiae HAC1i cells

The unfolded protein response (UPR) is a cytoprotective gene induction response that is induced upon dysfunction of the endoplasmic reticulum (ER), namely ER stress. In yeast Saccharomyces cerevisiae cells, the UPR is triggered by the ER-located transmembrane endoribonuclease Ire1 that, in response to ER stress, splices the HAC1 mRNA to yield the HAC1i mRNA, which is translated into an active transcription factor being responsible for the UPR. Therefore, the UPR is constitutively induced even without ER stress stimuli when cells carry a mutation to have the spliced HAC1i sequence on the HAC1 locus (HAC1i cells). While HAC1i cells grow fairly slowly, we obtained a spontaneous mutant of HAC1i cells that grew rapidly.