The Candida albicans CUG-decoding ser-tRNA has an atypical anticodon stem-loop structure

In many Candida species, the leucine CUG codon is decoded by a tRNA with tw o unusual properties: it is a ser-tRNA and, uniquely, has guanosine at posi tion 33 (G33). Using a combination of enzymatic (V-1, RNase, RnI nuclease) and chemical (Pb2+, imidazole) probing of the native Candida albicans ser-t RNA(CAG), we demonstrate that the overall tertiary structure of this tRNA r esembles that of a ser-tRNA rather than a leu-tRNA, except within the antic odon arm where there is considerable disruption of the anticodon stem. Usin g non-modified in vitro transcripts of the C. albicans ser-tRNA(CAG), carry ing G, C, U or A at position 33, we demonstrate that it is specifically a G residue at this position that induces the atypical anticodon stem structur e. Further quantitative evidence for an unusual structure in the anticodon arm of the G33-tRNA is provided by the observed change in kinetics of methy lation of the G at position 37, by purified Escherichia coli m(1)G37 methyl transferase. We conclude that the anticodon arm distortion, induced by a gu anosine base at position 33 in the anticodon loop of this novel tRNA, resul ts in reduced decoding ability which has facilitated the evolution of this tRNA without extinction of the species encoding it. (C) 1999 Academic Press .