RNA interference (RNAi) involves the targeting and silencing of cognate transcripts by small RNAs. This pathway has been found to function in gene expression regulation across species and indeed Kingdoms, but there are some key organisms in which RNAi has not yet been found. The budding yeasts, including Saccharomyces cerevisiae, have been considered to be a set of species in which this pathway is not present, whereas the fission yeast, Schizosaccharomyces pombe, has an active RNAi pathway involved in heterochromatin formation and centromeric silencing. By examining homologs of key RNAi pathway components in species closely related to S. cerevisiae, Bartel and colleagues (Science Express doi:10.1126/science.1176945, published online 10 September 2009) have now found that some budding yeasts do in fact have this silencing pathway, although it has been lost, as previously suggested, in S. cerevisiae.
Having provided genetic and biochemical evidence for the functionality of the enzymes needed for RNAi, the authors carried out a sort of 'reverse evolution' experiment and expressed the S. castellii AGO1 and DCR1 genes in S. cerevisiae. These genes are sufficient to generate an S. cerevisiae strain that can now mediate RNAi. Futhermore, in the RNAi-competent S. cerevisiae strain, endogenous retrotransposons are silenced.
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