The snR13 gene was PCR amplified using primers snR13fw (5΄ AATAGGATCCCAACGTGAAGAAGCC 3΄) and snR13rev (5΄ AACAGAATTCGCTTGCTTAGGCCCAAC 3΄) and was cloned into the Acc65I/XhoI sites of pRS416. Plasmids expressing wild-type or mutant snR13 were then transformed into the yeast strain YS630 (ade2, his3, trp1, ura3, leu2, snR13::TRP) (27). Mutations in the snR13 coding sequence were generated using PCR-based mutagenesis.