TRANSCRIPTIONAL REMODELING AND G(1) ARREST IN DIOXYGEN STRESS IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae, which lack a functional SOD1 gene, encoding the cytosolic Cu,Zn-superoxide dismutase (SOD1), exhibit a variety of metabolic defects in aerobic but not in anaerobic growth. We test here the hypothesis that some of these defects may be due to specific tran scriptional changes programmed for cell survival under dioxygen stress . Analysis of the budding pattern and generation time showed that the slower proliferation of an sod1 Delta mutant strain under air was due to an increase from 42 to 89 min spent in the G(1) phase of the cell c ycle. This delay in G(1) was not due to an overall decline in biosynth etic activity since total protein and mRNA synthesis was not reduced e ven under 100% O-2. However, rRNA synthesis was strongly decreased, e. g. by 80% in the mutant under 100% O-2 (in comparison to N-2). Under t hese conditions, the mutant permanently arrested in G(1); this arrest was due to an inhibition of the Start function that prepares yeast for S phase. This Start arrest was due to an inhibition of transcription of the autoregulated G(1) cyclins, CLN1 and CLN2; the transcription of the constitutive G(1) cyclin, CLN3, was unaffected by the stress. Exp ression of a hyperstable Cln3 prevented the G(1) arrest, indicating th at it was due solely to the inhibition of cell cycle-dependent cyclin expression. This remodeling of transcription in oxidative stress was s een also in the inhibition of glucose derepression of SUC2 expression. In contrast, the signaling and activation of mating pheromone (FUS1) and copper-responsive (CUP1) promoter activity were not affected by di oxygen stress, while genes encoding other anti-oxidant enzymes (SOD2, CTT1 and CTA1) were strongly induced. The UBI loci, encoding ubiquitin , were particularly good examples of this pattern of negative and posi tive transcriptional response to the stress. UBI1-UBI3 expression was repressed in the mutant under 100% O-2, while expression of UBI4 was s trongly induced. The data demonstrate that extensive remodeling of tra nscription occurs in yeast under a strong dioxygen stress. This remode ling results in a pattern of expression of gene products needed for de fense and repair, and suppression of activities associated with normal proliferative growth.