A stress-blinded Atf1 favors heterochromatin formation but impairs directionality of mat2/3 switching

The MAP kinase Sty1 phosphorylates and activates the transcription factor Atf1 in response to several stress conditions, which then shifts from a transcriptional repressor to an activator. Atf1 also participates in heterochromatin assembly at the mat locus, in combination with the RNA interference (RNAi) machinery. Here, we study the role of signal-dependent phosphorylation of Atf1 in heterochromatin establishment at mat, using different Atf1 phospho mutants. While a hypo-phosphorylation Atf1 mutant, Atf1.10M, mediates heterochromatin assembly, a phosphomimic Atf1.10D is unable to maintain silencing. In a minimal mat locus, lacking the RNAi-recruiting cis elements and displaying intermediate silencing, Atf1.10M restores full heterochromatin assembly capacity. However, evolution experiments with this stress-blinded Atf1.10M show that it is unable to facilitate switching between the donor site mat3 and mat1. We propose that the unphosphorylated, inactive Atf1 contributes to proper heterochromatin assembly by recruiting repressive complexes, but its stress-dependent phosphorylation is required for recombination/switching to occur. The plasticity of a transcription factor explains its contribution to apparent antagonistic processes, such as chromatin silencing and accessibility to the large DNA recombination machinery.

丝裂原活化蛋白激酶Sty1在多种应激条件下对转录因子Atf1进行磷酸化并激活其活性，此时Atf1从转录抑制因子转变为激活因子。Atf1亦参与异染色质在mat位点的组装，与RNA干扰（RNAi）机制协同作用。本研究旨在探讨Atf1信号依赖性磷酸化在mat位点异染色质建立中的作用，采用不同的Atf1磷酸化突变体进行研究。在低磷酸化突变体Atf1.10M中，异染色质组装得以介导，而模拟磷酸化的Atf1.10D则无法维持沉默状态。在缺乏RNAi招募顺式元件且呈现中等沉默状态的简化mat位点中，Atf1.10M恢复了完整的异染色质组装能力。然而，通过进化实验对这种应激盲的Atf1.10M进行测试表明，其无法促进供体位点mat3与mat1之间的转换。我们提出，未磷酸化、非活性的Atf1通过招募抑制复合物，有助于异染色质组装的正确进行，但其应激依赖性磷酸化对于重组/转换的发生是必需的。转录因子的可塑性解释了其在看似对抗性过程（如染色质沉默和对大型DNA重组机制的开放性）中的贡献。