Participation of the intron in the reaction catalyzed by the Xenopus tRNA splicing endonuclease.

Introns have generally been assumed to be passive in the transfer RNA splicing reaction. Experiments have now been done showing that the endonuclease is able to cut a precursor provided that a base in the single-stranded loop of the intron can pair with the base of the 5' exon situated at the position that immediately follows the anticodon stem (position 33 in the yeast tRNA isoacceptor pre-tRNA(Leu)3, position 32 in yeast pre-tRNA(Phe)). The elucidation of the role of the intron reveals that in addition to the conserved bases, there are positions in the mature domain which, although not necessarily occupied by the same base in all pre-tRNA's, nevertheless have a fundamental role in the splicing reaction. These positions are termed cardinal positions.