Nuclear retention of fission yeast dicer is a prerequisite for RNAi-mediated heterochromatin assembly

RNAseIII ribonucleases act at the heart of RNA silencing pathways by processing precursor RNAs into mature microRNAs and siRNAs. In the fission yeast Schizosaccharomyces pombe, siRNAs are generated by the RNAseIII enzyme Dcr1 and are required for heterochromatin formation. In this study, we have analyzed the subcellular localization of Dcr1 and found that it accumulates in the nucleus and is enriched at the nuclear periphery. Nuclear accumulation of Dcr1 depends on a short motif which constrains nuclear export promoted by the double-stranded RNA binding domain of Dcr1. Absence of this motif renders Dcr1 mainly cytoplasmic and is accompanied by remarkable changes in gene expression and failure to assemble heterochromatin. Our findings suggest that dicer proteins are shuttling proteins and that the steady-state subcellular levels can be shifted towards either compartment. This has implications for the mechanism of RNAi-mediated heterochromatin assembly and the spatial organization of RNA silencing pathways in general. 25 chips representing 6 different strains. Every condition is represented by at least two replicates