Characterization of serine and leucine tRNAs in an asporogenic yeast Candida cylindracea and evolutionary implications of genes for tRNACAG responsible for translation of a non-universal genetic code

Five serine and three leucine isoacceptor tRNAs were purified from the asporogenic yeast Candida cylindracea, in which codon CUG is translated as serine Instead of leucine [1], and their primary structures were determined. From the wobble hypothesis [2], it was assumed that one of the tRNA species (Leu1), with the antlcodon CmAA, corresponded to the UUG leucine codon, and that the remaining two leucine tRNAs (Leu2 and Leu3), with the same IAG antlcodon sequence, would decode the CUU, CUC and CUA codons as leucine, but not the CUG codon ; this was clarified by an In vitro translation experiment with C.cylindracea using synthetic mRNAs containing the CUA or CUG codons. One of the serine tRNAs (Ser1) has already been demonstrated to have the antlcodon CAG and to be responsible for translation of the codon CUG In C.cylindracea [3]. Three of the other species of tRNA^Sert.S and 4), with the anticodon sequences cmUGA, IGA and CGA, can translate all four codons in the UCN codon box, while the remaining species (Ser5), with the anticodon GCU, corresponds to AGU and AGC serine codons. The gene sequences for these five serine and three leucine tRNAs were also determined, with the finding that only tRNA^CAG (Ser1) has an Intron. At least five different types of tRNACAG genes exist in the genome of C.cylindracea. The nucleotide sequences of the flanking regions of these tRNACAG genes indicated that the tRNA^CAG gene has duplicated at least three times on the genome. The existence of multiple genes for tRNACAG on the genome may account for the observation that codon CUG is used very frequently in C.cylindracea. All of these tRNA^CAG genes contain the CCA sequence in their 3' termini, suggesting the possibility that during their multiplication process In the evolution of the C.cylindracea genome, the tRNACAG molecule was Integrated into DNA via reverse transcription.