A single Argonaute protein mediates both transcriptional and posttranscriptional silencing in Schizosaccharomyces pombe

The Schizosaccharomyces pombe genome encodes only one of each of the three major classes of proteins implicated in RNA silencing: Dicer (Dcr1), RNA-dependent RNA polymerase (RdRP; Rdp1), and Argonaute (Ago1). These three proteins are required for silencing at centromeres and for the initiation of transcriptionally silent heterochromatin at the mating-type locus. Here, we show that the introduction of a double-stranded RNA (dsRNA) hairpin corresponding to a green fluorescent protein (GFP) transgene triggers classical RNA interference (RNAi) in S. pombe. That is, GFP silencing triggered by dsRNA reflects a change in the steady-state concentration of GFP mRNA, but not in the rate of GFP transcription. RNAi in S. pombe requires dcr1, rdp1, and ago1, but does not require chp1, tas3, or swi6, genes required for transcriptional silencing. Thus, the RNAi machinery in S. pombe can direct both transcriptional and posttranscriptional silencing using a single Dicer, RdRP, and Argonaute protein. Our findings suggest that these three proteins fulfill a common biochemical function in distinct siRNA-directed silencing pathways.