Nucleotide sequence of genes coding for tRNAPhe and tRNATyr from a repeating unit of X. laevis DNA.

A cloned 3.18 kb repeating unit of Xenopus laevis DNA, known to contain two tRNA1Met genes, has been partially sequenced. The sequenced region could potentially code for two more tRNA species. One is a phenylalanine tRNA with the same primary sequence as that found in a variety of mammals. The other is a tyrosine tRNA with a 13 bp intervening sequence located immediately adjacent or very close to the 3' nucleotide of the anticodon. The two putative genes have opposite polarity and the sequences encoding the 5' nucleotides of the mature tRNAs are separated by only 72 bp of DNA. Homologous GC-rich sequences of dyad symmetry precede each gene. Neither gene codes for the 3' terminal CCA expected in the nature tRNAs. A short T cluster is present in the noncoding strand 3 bp away from the 3' end of the tRNATyr gene. A similar short T cluster is located 26 bp downstream of the tRNAPhe sequence. The nucleotides in between are very GC-rich and are capable of forming a stable RNA hairpin. The possible significance of these features for tRNA gene evolution, RNA splicing and transcription is discussed.