RID is required for both repeat-induced point mutation and nucleation of a novel transitional heterochromatic state for euchromatic repeats.

To maintain genome integrity, 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 (R-L-R) 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 additionally requiring DIM-2. The differences between mechanisms associated with older and newer duplicates caution against extrapolation from mechanisms inferred from model experimental systems. Additionally, while chromatin status affects RIP, we also show that RID, when tethered with LexA, acts as a nucleation center for the transition from euchromatin to heterochromatin in an HDA-1 dependent fashion. Constitutive heterochromatin by contrast is largely HDA1 independent and depends on HDA-1 paralogs. RID is thus a dual function initiator of both RIP and the transition to heterochromatin.