Bovine mitochondrial tRNASer(UCN) has been thought to have two U-U mismatches at the top of the acceptor stem, as inferred from its gene sequence. However, this unusual structure has not been confirmed at the RNA level. In the course of investigating the structure and function of mitochondrial tRNAs, we have isolated thebovine liver mitochondrial tRNASer(UCN) and determined its complete sequence including the modified nucleotides. Analysis of the 5′-terminal nucleotide and enzymatic determination of the whole sequence of tRNASer(UCN) revealed that the tRNA started from the third nucleotide of the putative tRNASer(UCN) gene, which had formerly been supposed. Enzymatic probing of tRNASer(UCN) suggests that the tRNA possesses an unusual cloverleaf structure with the following characteristics. (1) There exists only one nucleotide between the acceptor stem with 7 base pairs and the D stem with 4 base pairs. (2) The anticodon stem seems to consist of 6 base pairs. Since the same type of cloverleaf structure as above could be constructed only for mitochondrial tRNASer(UCN) genes of mammals such as human, rat and mouse, but not for those of nonmammals such as chicken and frog, this unusual secondary structure seems to be conserved only in mammalian mitochondria.
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