Poster abstracts

Poster number 31 submitted by Gina Nostramo

tRNA introns: A novel class of small non-coding regulatory RNAs

Gina Nostramo (Department of Molecular Genetics), Anita K. Hopper (Department of Molecular Genetics)

Abstract:
A subset of genes encoding tRNAs contain introns. In most species, tRNA introns are located 3’ to the anticodon and are removed by the tRNA splicing endonuclease. Pre-tRNA splicing is essential in most organisms because, for at least one tRNA family, all reiterated tRNA genes contain an intron. Thus, the genome cannot be decoded without tRNA splicing. Why has the possession of tRNA introns been conserved from archaea to vertebrates? One possibility is that released introns function as small non-coding regulatory RNAs by base-pairing with mRNAs. To test this hypothesis, we searched all annotated ORFs in S. cerevisiae for sequences with complementarity to tRNA introns, using the criteria of at least 13 nts perfect complementarity or 14/15 nts allowing for 1 substitution. We identified 32 ORFs with complementarity to the 60 nt tRNAIleUAU intron. To test the role of this intron on gene products with complementarity, we generated a strain lacking tRNAIle introns (IleiΔ) and assessed changes in mRNA levels of the 32 ORFs relative to wild-type cells, as compared to changes in mRNA levels of 28 randomly selected ORFs. In the IleiΔ strain, mRNA levels are significantly increased for genes with complementarity to the tRNAIle intron versus those without, suggesting that tRNAIle introns may function as inhibitors of sequence-specific gene expression/mRNA turnover. An additional role for the tRNAIle intron was also identified. Protein, but not mRNA, levels of GFP-tagged Orc2, whose ORF contains 14 nts perfect complementarity to the tRNAIle intron, are decreased in an IleiΔ strain, even though protein levels of genes lacking complementarity are unaltered. This suggests that the tRNAIle intron may also act in a sequence-specific manner to positively regulate Orc2 levels. Overall, our data support the exciting possibility that tRNA introns are the newest member of non-coding regulatory RNAs, functioning as positive and negative regulators of gene expression.

Keywords: tRNA, Introns, non-coding RNAs