For euchromatic repeats RID is required for both Repeat Induced Point mutation and nucleation of a novel transitional heterochromatic state

Gene inactivation via heterochromatization is important for genome integrity and prevention of unwanted transcripts with dysregulation in cancer. Repeat sequences are subject to heterochromatin inactivation and, in Neurospora, Repeat-Induced-Point mutation (RIP). The initiating factors behind both are poorly understood. We resolve the paradoxical observation that newly introduced Repeat-Linker-Repeat (RLR) constructs require RID alone for RIP, while genomic repeats are RIPed in the absence of RID, showing that eu- and hetero- chromatic repeats are handled differently, the latter requiring DIM-2 as well. The differences between mechanisms associated with older and newer duplicates caution against extrapolation from mechanisms inferred from model experimental systems to all genomically relevant processes. Additionally, while chromatin status affects RIP, RID we also show via lexA tethering is the nucleation centre for a transition from eu- to hetero- chromatin, in an HDA-1 dependent fashion. Constitutive heterochromatin by contrast is HDA1 independent and depends on HDA-1 paralogs. RID is thus a dual function initiator of both RIP and the transition to heterochromatin.