Distinct regulatory proteins control the graded transcriptional response to increasing H2O2 levels in the fission yeast, S. pombe

The signaling pathways that sense adverse stimuli and communicate with the nucleus to initiate appropriate changes in gene expression are central to the cellular stress response. Here, we have characterized the role of the Sty1 (Spc1) stressactivated MAPK pathway, and the Pap1 and Atf1 transcription factors, in regulating the response to H2O2 in the fission yeast, Schizosaccharomyces pombe. We find that H2O2 activates the Sty1 pathway in a dose-dependent manner via at least two sensing mechanisms. At relatively low levels of H2O2, a two component-signaling pathway, which feeds into either of the two stress-activated MAPKKKs, Wak1 or Win1, regulates Sty1 phosphorylation. In contrast, at high levels of H2O2, Sty1 activation is controlled predominantly by a two-component independent mechanism and requires the function of both Wak1 and Win1. Individual transcription factors were also found to function within a limited range of H2O2 concentrations. Pap1 activates target genes primarily in response to low levels of H2O2 while Atf1 primarily controls the transcriptional response to high concentrations of H2O2. Our results demonstrate that S. pombe uses a combination of stress responsive regulatory proteins to gauge and effect the appropriate transcriptional response to increasing concentrations of H2O2.