The primary structures of three yeast mitochondrial serine tRNA isoacceptors

Yeast mitochondria contain several isoaccepting species of serine-tRNA. The relative amount of these isoacceptors varies according to the conditions used to grow the yeast cells. In order to gain insight into the structural differences among these isoacceptors, the three mitochondrial tRNAsSer, which are present in derepressed yeast cells, have been sequenced. The primary structure of tRNASer1 differs considerably from that of tRNASer2; these two isoacceptors have only 39 nucleotides in common. In contrast, tRNASer3 differs from tRNASer2 by only one post-transcriptional modification: the psi residue in position 28 of tRNASer2 is replaced by a normal U in tRNASer3. Unlike tRNASer2 and tRNASer3, the primary sequence of tRNASer1 shows two unusual structural features: it has a D in position 14 instead of the "universal" A14 of the standard tRNA cloverleaf and it contains two G residues between the D-stem and the anticodon-stem. Considering their respective anticodons, tRNASer1 should recognize the two serine codons A-G-C and A-G-U, whereas both tRNASer2 and tRNASer3 should recognize all four serine codons of the U-C-N series.