Effect of intron mutations on processing and function of Saccharomyces cerevisiae SUP53 tRNA in vitro and in vivo.

The Saccharomyces cerevisiae leucine-inserting amber suppressor tRNA gene SUP53 (a tRNALeu3 allele) was used to investigate the relationship between precursor tRNA structure and mature tRNA function. This gene encodes a pre-tRNA which contains a 32-base intron. The mature tRNASUP53 contains a 5-methylcytosine modification of the anticodon wobble base. Mutations were made in the SUP53 intron. These mutant genes were transcribed in an S. cerevisiae nuclear extract preparation. In this extract, primary tRNA gene transcripts are end-processed and base modified after addition of cofactors. The base modifications made in vitro were examined, and the mutant pre-tRNAs were analyzed for their ability to serve as substrates for partially purified S. cerevisiae tRNA endonuclease and ligase. Finally, the suppressor function of these mutant tRNA genes was assayed after their integration into the S. cerevisiae genome. Mutant analysis showed that the totally intact precursor tRNA, rather than any specific sequence or structure of the intron, was necessary for efficient nonsense suppression by tRNASUP53. Less efficient suppressor activity correlated with the absence of the 5-methylcytosine modification. Most of the intron-altered precursor tRNAs were successfully spliced in vitro, indicating that modifications are not critical for recognition by the tRNA endonuclease and ligase.