A phospho-mimicking Atf1 mutant bypasses the transcription activating function of the MAP kinase Sty1

The MAP kinase Sty1 of fission yeast accumulates in the nucleus upon stress, and stimulates expression of hundreds of genes, including atf1, via the constitutively nuclear transcription factor Atf1. The role on transcription of the signal-dependent phosphorylation of Atf1 by Sty1 is unclear. We have expressed the transcription factor and mutant derivatives from a constitutive promoter, to unchain Atf1 activity from its protein levels. Expression of a non-phosphorylatable Atf1 renders cells sensitive to oxidative stress, due to the impaired transcription of a subset of genes, those dependent on another transcription factor, Pap1. Cells expressing a phospho-mimicking mutant display enhanced resistance to stress, and while expression of the Pap1-dependent genes is still stress-dependent, a second subset of genes is constitutively expressed. Our study demonstrates that phosphorylation of Atf1 by Sty1 does not stimulate binding of the transcription factor to DNA but rather establishes a platform of interaction with the transcriptional machinery. Overall design: We report the differences in gene expression of cells expressing either phosphorylated or unphosphorylatable form of Atf1 in basal and after hydrogen peroxide stress.