Wheat germ splicing endonuclease is highly specific for plant pre-tRNAs

Intron-containing pre-tRNAs from organisms as different as yeast, Nicotiana, Xenopus and man are efficiently spliced and processed in a HeLa cell extract. They are also correctly processed in a wheat germ extract; however, the intron is removed only from the tobacco pre-tRNA. To determine whether plant pre-tRNA introns have any specific structural and/or sequence feature we have cloned two intron-containing tRNATyr genes from the plant Arabidopsis. Comparison of these genes, of the Nicotiana tRNATyr gene and of a Glycine max tRNAMet gene reveals that plant introns from three different species have no sequence homology and are only 11 to 13 nucleotides long. Thus, short length may be one important feature of plant introns. Furthermore, the 5' and 3' splice sites are separated by 4 bp in the extended anticodon stems of these pre-tRNA structures. In contrast, yeast and vertebrate introns are rather variable in length and the splice sites are separated by 5 or 6 bp. These differences in distance and relative helical orientation of the splice sites in plant pre-tRNAs versus pre-tRNAs from other organisms are obviously tolerated by the vertebrate splicing endonuclease, but not at all by the plant enzyme.