Nuclear processing and export of microRNAs in Arabidopsis

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Abstract
In mammalian cells, the nuclear export receptor, Exportin 5 (Exp5), exports pre-microRNAs (pre-miRNAs) as well as tRNAs into the cytoplasm. In this study, we examined the function of HASTY (HST), the Arabidopsis ortholog of Exp5, in the biogenesis of miRNAs and tRNAs. In contrast to mammals, we found that miRNAs exist as single-stranded 20- to 21-nt molecules in the nucleus in Arabidopsis. This observation is consistent with previous studies indicating that proteins involved in miRNA biogenesis are located in the nucleus in Arabidopsis. Although miRNAs exist in the nucleus, a majority accumulate in the cytoplasm. Interestingly, loss-of-function mutations in HST reduced the accumulation of most miRNAs but had no effect on the accumulation of tRNAs and endogenous small interfering RNAs, or on transgene silencing. In contrast, a mutation in PAUSED (PSD), the Arabidopsis ortholog of the tRNA export receptor, Exportin-t, interfered with the processing of tRNA-Tyr but did not affect the accumulation or nuclear export of miRNAs. These results demonstrate that HST and PSD do not share RNA cargos in nuclear export and strongly suggest that there are multiple nuclear export pathways for these small RNAs in Arabidopsis.