Minimum intron requirements for tRNA splicing and nuclear transport in Xenopus oocytes.

The presence or absence of an intron defines two classes of eukaryotic nuclear tRNA genes whose transcripts differ in a requirement for splicing. Using quantitative nuclear microinjection, we have previously found that nucleocytoplasmic transport of these two classes of tRNAs involves pathways which differ in one or more limiting components. To examine substrate features which distinguish these two pathways, a series of variants of a Xenopus tRNA(Tyr) gene were constructed in which the intron size was altered. The splicing and transport properties of the resulting transcripts were examined in oocyte microinjection and in vitro processing assays. The addition of one or two nucleotides at the splice site equivalent in an intronless gene produced transcripts which could be transported without splicing. However, transport was reduced relative to the mature-sequence tRNA, suggesting the anticodon loop (interrupted in pre-tRNAs) may be recognized by the intronless tRNA transport apparatus. Transcripts with four- or six-nucleotide intervening sequences were incompletely spliced with cleavage at only the 3' splice site. Neither unspliced precursor nor partially processed intermediates were efficiently transported. The results of coinjection experiments using tRNA and pre-tRNA competitors suggest that simple retention by the splicing apparatus may not account for failure to export these RNAs. Finally, a requirement for splicing is not unique to transport of pre-tRNA(Tyr) since a pre-tRNA(3Leu) variant which was not spliced was also not exported.