Unique structure of new serine tRNAs responsible for decoding leucine codon CUG in various Candida species and their putative ancestral tRNA genes.

In an asporogenic yeast, Candida cylindracea, codon CUG is not translated as leucine but as serine. On the basis of our recent work on the determination of the genetic code using in vitro translation systems coupled with isolation of the corresponding tRNA molecules, it appears that this non-universal genetic code is unitized not only in C cylindracea but also in various Hemiascomycetes. Here we show that in addition to the species already reported, three pathogenic yeasts, C guilliermondii, C lusitaniae and C tropicalis, have tRNA(Ser)CAG, indicating that this non-universal genetic code (CUG=Ser) also exists in these species. Determination of their primary structures revealed that the uridine conserved at position 33 in usual tRNAs, is replaced by guanosine or cytidine. This suggests that the three-dimensional structures of the anticodon loop of these tRNAs differ from the conventional structure comprising the U turn in this position. Moreover, we succeeded in isolating putative ancestral serine tRNA genes whose sequences are highly homologous to tRNA(Ser)CAG in each case. These tRNA genes all have the anticodon sequence CGA corresponding to the codon UCG, indicating that tRNA(Ser)CAG might have emerged from tRNA(Ser)CGA during evolutionary change of the assignment of codon CUG.