RNAi-independent heterochromatin nucleation by the stress-activated ATF/CREB family proteins.

At the silent mating-type interval of fission yeast, the RNA interference (RNAi) machinery cooperates with cenH, a DNA element homologous to centromeric repeats, to initiate heterochromatin formation. However, in RNAi mutants, heterochromatin assembly can still occur at low efficiency. Here, we report that Atf1 and Pcr1, two ATF/CREB family proteins, act in a parallel mechanism to the RNAi pathway for heterochromatin nucleation. Deletion of atf1 or pcr1 alone has little effect on silencing at the mating-type region, but when combined with RNAi mutants, double mutants fail to nucleate heterochromatin assembly. Moreover, deletion of atf1 or pcr1 in combination with cenH deletion causes loss of silencing and heterochromatin formation. Furthermore, Atf1 and Pcr1 bind to the mating-type region and target histone H3 lysine-9 methylation and the Swi6 protein essential for heterochromatin assembly. These analyses link ATF/CREB family proteins, involved in cellular response to environmental stresses, to nucleation of constitutive heterochromatin.