Our paper is the first to show that processing of microRNA precursors in Arabidopsis likely occurs in the nucleus through the action of nuclear localized DICER-LIKE 1 (DCL1), which is one of four DCL proteins encoded in the Arabidopsis genome. This differs from the situation in mammals where the single Dicer enzyme processes microRNA precursors in the cytoplasm.

It describes a new discovery but also a somewhat new technique: to bolster our conclusion that processing of microRNA precursors (and at least some siRNA precursors) occurs in the nucleus, we engineered a nuclear-localized version of the viral protein P19. This protein suppresses RNAi by acting as a size-selective caliper to sequester short RNAs from the silencing machinery. The nuclear-localized P19—but not the cytoplasmic P19—reduced accumulation of miRNAs and a subset of siRNAs.

Plant and animal development depends on selective gene repression by microRNAs. In plants, these tiny RNAs appear to be produced in the cell nucleus, whereas in mammals, they are generated outside the nucleus.

We have studied RNA-mediated silencing for many years in our lab. We were interested in identifying which of the four DCL activities in Arabidopsis was required to produce short RNAs from a double-stranded RNA encoded by a DNA inverted repeat. We suspected that processing might occur in the nucleus, and our results provided support for this hypothesis.