Adaptation to stress triggers the most dramatic shift in gene expression in fission yeast (Schizosaccharomyces pombe), and this response is driven by signaling via the MAPK Sty1. Upon activation, Sty1 accumulates in the nucleus, and stimulates expression of hundreds of genes via the nuclear transcription factor Atf1, including expression of atf1itself. However, the role of stress-induced, Sty1-mediated Atf1 phosphorylation in transcriptional activation is unclear. To this end, we expressed Atf1 phosphorylation mutants from a constitutive promoter, to uncouple Atf1 activity from endogenous, stress-activated Atf1 expression. We found that cells expressing a nonphosphorylatable Atf1 variant are sensitive to oxidative stress because of impaired transcription of a subset of stress genes whose expression is controlled also by another transcription factor, Pap1. Furthermore, cells expressing a phosphomimicking Atf1 mutant display enhanced stress resistance, and while expression of the Pap1-dependent genes still relied on stress induction, another subset of stress-responsive genes was constitutively expressed in these cells. We also observed that in the cells expressing the phosphomimicking Atf1 mutant, the presence of Sty1 is completely dispensable, with all stress defects of Sty1-deficient cells being suppressed by expression of the Atf1 mutant. We further demonstrated that Sty1-mediated Atf1 phosphorylation does not stimulate binding of Atf1 to DNA, but rather establishes a platform of interactions with the basal transcriptional machinery to facilitate transcription initiation. In summary, our results provide evidence that Atf1 phosphorylation by the MAPK Sty1 is required for oxidative stress responses in fission yeast cells and by promoting transcription initiation.

This work was supported by the Ministerio de Economía y Competitividad (Spain), PLAN E and FEDER (BFU2015-68350-P to EH), and by 2014-SGR-154 from Generalitat de Catalunya (Spain) to EH. LSM is recipient of a Juan de la Cierva post-doctoral contract, and CSC is recipient of a María de Maeztu pre-doctoral fellowship, both by the Ministerio de Economía y Competitividad (Spain). EH is recipient of an ICREA Academia Award (Generalitat de Catalunya, Spain).