All human tRNATyr genes contain introns as a prerequisite for pseudouridine biosynthesis in the anticodon.

Two synthetic oligonucleotides, one specific for the 5' exon, the other spanning the splice junction, were used to show that (a) the human haploid genome contains at least 12 independent gene loci for tRNATyr, and (b) that all of them carry an intron. From one of the cloned human tRNATyr genes (pHtT1) the 20 bp intron was deleted to generate pHtT1 delta. Homologous in vitro transcription, fingerprint analyses of the products and elucidation of their nucleoside composition revealed that the pseudouridine (psi 35) in the center of the anticodon of tRNATyr was synthesized in the intron-containing precursor whereas this U to psi modification did not take place in precursors or mature tRNATyr derived from pHtT1 delta. On the basis of these results and of studies from other laboratories we suggest that the evolutionary pressure for maintaining introns in eukaryotic tRNAsTyr is this strict intron-requirement for psi 35 synthesis. Taking into account that all eukaryotic cytoplasmic tRNAsTyr contain a psi 35 we discuss here a special need for this modified nucleoside in stabilizing codon-anticodon interactions involving (a) classical base pairing upon translation of tyrosine codons and (b) unconventional interactions during UAG amber codon suppression by tRNATyrG psi A in eukaryotic cells.