ABTRACTThe heterochromatin spreading reaction is a central contributor to the formation of gene-repressive structures, which are re-established with high fidelity following replication. The high fidelity of this process is not obviously encoded in the primary spreading reaction. To resolve origins of stable inheritance of repression, we probed the intrinsic fidelity of spreading events in fission yeast using a system that quantitatively describes the spreading reaction in live single cells. We show that spreading from RNAi-nucleated elements is stochastic, multimodal, and fluctuates dynamically across time. In contrast, a second form of spreading, nucleated by the cis-acting element REIII, is deterministic, has high memory capacity and acts as the source of locus fidelity. REIII enables fidelity in part by endowing the locus with resistance to perturbations. Together, our results suggest that epigenetic capacity may not be intrinsically encoded in the spreading reaction, but rather requires collaboration with specialized memory elements.This movie consists of imaging in 4 channels, listed from top to bottom: Bright field, "green", "orange", and "red" for cell #407 from the strain PAS391 ΔREIII HSS . X-Y fluorescence plot for this cell is shown in Figure 3D.
SVideo 5: Cell #123 from strain PAS387This movie consists of imaging in 4 channels, listed from top to bottom: Bright field, "green", "orange", and "red" for cell #123 from the strain PAS387 ΔK HSS . X-Y fluorescence plot for this cell is shown in Figure 3E.