Toward unraveling biogenesis of Dicer-independent priRNAs and siRNAs in Schizosaccharomyces pombe

RNA interference (RNAi) is a highly conserved process of gene
silencing in which Argonaute family proteins are guided by small
RNA molecules to complementary targets. In the fission yeast
Schizosaccharomyces pombe, RNAi is required for heterochromatin
formation at centromeres. Although it seems counterintuitive,
pericentromeric heterochromatin in fission yeast is transcribed.
The transcripts are processed by RNAi machinery, which is in turn
guided back to the pericentromeric repeats by sequence
complementarity of the Argonaute-bound small interfering RNA
(siRNA) and the nascent transcript. This generates a
positive-feedback loop of siRNA amplification that recruits factors
required for the assembly of heterochromatin. Previously, it was
suggested that a fission yeast class of Dicer-independent small
RNAs called primal small RNAs (priRNAs) initiates the
positive-feedback loop of siRNA generation and heterochromatin
assembly. However, the biogenesis of priRNAs as well as of
Dicer-independent small RNAs from other organisms was not well
understood. The results presented here identify Triman, a novel
3’-5’ exonuclease that is involved in the final step of biogenesis
of both priRNAs and siRNAs in fission yeast. It was observed that
Argonaute binds longer priRNA and siRNA precursors from the total
RNA fraction. This is followed by the recruitment of Triman to trim
3’ ends of Argonaute-bound small RNAs to the mature size. The final
trimming of priRNAs and siRNAs is required for de novo
heterochromatin formation at centromeres and the mating-type locus
as well as for the maintenance of facultative heterochromatin
islands. Furthermore, it was shown that in cells lacking Rrp6, a
nuclease subunit of the exosome, RNAi targets various genes across
the yeast genome. This demonstrated that the exosome protects the
genome against aberrant RNAi. Spurious RNAi targeting in rrp6∆
cells at majority of loci occurs via accumulation of antisense
transcripts that are processed into priRNAs in a Triman-dependent
manner. These results suggest that Argonaute association with
cellular degradation products which are processed into priRNAs
might serve as a surveillance mechanism to guard the genome against
invading genomic elements (Marasovic et al. 2013).