Unbiased screen of RNA tailing activities reveals a poly(UG) polymerase

Ribonucleotidyl transferases (rNTases) add non-templated ribonucleotides to diverse RNAs. We developed TRAID-Seq, a screening strategy in S. cerevisiae to identify sequences added to a reporter RNA at single-nucleotide resolution by overexpressing candidate enzymes from different organisms. The rNTase activities of 22 previously unexplored enzymes were determined. In addition to poly(A)- and poly(U)-adding enzymes, we identified a C-adding enzyme that is likely part of a two-enzyme system that adds CCA to tRNAs in a eukaryote; a nucleotidyl transferase that adds nucleotides to RNA without apparent nucleotide preference; and a poly(UG) polymerase, C. elegans MUT-2, which adds alternating U and G nucleotides to form poly(UG) tails. MUT-2 is known to be required for certain forms of RNA silencing, and mutations in the enzyme that are defective in silencing fail to add poly(UG) tails in our assay. We propose that MUT-2 poly(UG) polymerase activity is required to promote genome integrity and RNA silencing.