Impact of intron removal from tRNA genes on Saccharomyces cerevisiae

In eukaryotes and archaea, tRNA genes frequently contain introns, which are removed during maturation. However, biological roles of tRNA introns remain elusive. Here, we constructed a complete set of Saccharomyces cerevisiae strains in which the introns were removed from all the synonymous genes encoding 10 different tRNA species. All the intronless strains were viable, but the tRNA(Phe)(GAA) and tRNA(Tyr)(GUA) intronless strains displayed slow growth, cold sensitivity and defective growth under respiratory conditions, indicating physiological importance of certain tRNA introns. Northern analyses revealed that removal of the introns from genes encoding three tRNAs reduced the amounts of the corresponding mature tRNAs, while it did not affect aminoacylation. Unexpectedly, the tRNA(Leu)(CAA) intronless strain showed reduced 5.8S rRNA levels and abnormal nucleolar morphology. Because pseudouridine (Ψ) occurs at position 34 of the tRNA(Ile)(UAU) anticodon in an intron-dependent manner, tRNA(Ile)(UAU) in the intronless strain lost Ψ34. However, in a portion of tRNA(Ile)(UAU) population, position 34 was converted into 5-carbamoylmethyluridine (ncm(5)U), which could reduce decoding fidelity. In summary, our results demonstrate that, while introns are dispensable for cell viability, some introns have diverse roles, such as ensuring proper growth under various conditions and controlling the appropriate anticodon modifications for accurate pairing with the codon.