An RNAi-independent role of AP1-like stress response factor Pap1 in centromere and mating-type silencing in Schizosaccaromyces pombe .

Gene silencing in S. pombe occurs by heterochromatin formation at the centromere ( cen ), mating-type ( mat ) and telomere loci. It is mediated by silencing factors including Swi6, Clr1-4, Rhp6 and Pola. RNAi pathway also plays a role in establishment of silencing at the mat and cen loci. Recently, the stress response factors, Atf1 and Pcr1were shown to play an RNAi-independent role in silencing at the mat3 locus through a cis -acting Atf1-binding site located within the repression element REIII and recruitment of the silencing factors Clr3 and Clr6. Another cis -acting site, named repression element REII abutting the mat2 locus, also establishes heterochromatin structure through Clr5 and histone deacetylases but independently of H3-Lys9-methylation and RNAi. Here, we report the occurrence of binding sites for another oxidative response factor, the pombe AP1- like factor Pap1, at the mating-type, centromere and telomere loci. By genetic studies we show that these sites play a role in silencing at the outer repeats of centromeres as well as mating-type locus and this effect is mediated through Pap1 binding site and interaction with and recruitment of the HP1/Swi6. Importantly, pap1 Δ cells display a silencing defect even in absence of the oxidative stress. Such a role of Pap1 in heterochromatin formation may be evolutionarily conserved.