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 yeast, C guilliermondii, C lusitaniae and C tropicalis, have tRNASerCAG, 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 genesw hose sequences are highly homologous to tRNASerCAG in each case. These tRNA genes all have the anticodon sequence CGA corresponding to the codon UCG, indicating that tRNASerCAG might have emerged from tRNASerCGA during evolutionary change of the assignment of codon CUG.
ASJC Scopus subject areas