XRN1 is a Species-Specific Virus Restriction Factor in Yeasts

Paul A. Rowley; Brandon Ho; Sarah Bushong; Arlen Johnson; Sara L. Sawyer

To differentiate between models where Xrn1p restricts L-A through a passive 158 mechanism that is incidental to its inherant exonuclease activity, or through an active 159 mechanism where Xrn1p evolves to optimally suppress L-A replication, we first looked 160 for evidence of positive selection (dN/dS > 1) within the genes encoding the major 161 components of the SKI complex, the exosome, and Xrn1p ( Fig 1B) . Importantly, cerevisiae, S. paradoxus, S. mikatae, S. kudriavzevii, S. arboricolus, and S. bayanus) 171 [44] [45] [46] and created a multiple sequence alignment. We then analyzed each alignment 172 for evidence of codons with dN/dS > 1 using four commonly employed tests for positive 173 selection [47, 48] . We see some evidence for positive selection of specific codon sites in 174 several of these genes, however, only XRN1 and the exosome subunit gene RRP40 175 passed all four tests ( Fig 1C; Table S1 ). Other genes are determined to be under surprising for a critical and conserved gene involved in RNA quality control, but 203 consistent with the signatures of positive selection which suggest that certain parts of 204 this protein are highly divergent between species.