EXON SEQUENCE AND STRUCTURE REQUIREMENTS FOR TRANSFER-RNA SPLICING IN SACCHAROMYCES-CEREVISIAE

A survey of exon sequence and structure requirements for splicing was undertaken using labeled pre-tRNA substrates prepared by in vitro transcription of bacterial promoter-yeast tRNA(Tyr), gene fusions. Transcription templates were assembled from oligonucleotide cassettes allowing analysis of 22 derivatives affecting each of the potential secondary and certain tertiary interactions in the pre-tRNA. Effects on both excision of the intervening sequence by yeast endonuclease and joining of exons by ligase were examined. Replacements within the D- and T-stems and anticodon stems revealed that while the primary sequences of these segments were not essential for splicing, formation of base-paired structures was required. Replacements which altered the primary sequence while retaining the secondary structure of the aminoacyl stem allowed efficient excision by endonuclease but reduced joining by ligase. Potentially, the effects of changes within these stems may be indirect through effects on adjacent or overall structure. The presence of either structured or unstructured 5' leader and/or 3' trailer sequences had no effect on either splicing step. Alterations in the conserved Levitt tertiary pair (G15/C48), previously implicated in splicing of pre-tRNA(Phe), did not alter splicing of pre-tRNA(Tyr). A precursor in which the small (type I) extra arm in pre-tRNA(Tyr) was replaced with the large (type II) extra arm sequence from tRNA(Ser) was efficiently spliced. These and previous results suggest that only limited features of exon sequence or structure are recognized by the splicing enzymes.