Translation activity of mitochondrial tRNA with unusual secondary structure

Mammalian mitochondrial translation system requires two serine isoacceptor tRNAs, tRNA(Ser)GCU and tRNA(Ser)UGA, which correspond to codons AGY (Y = U or C) and UCN (N = A, G, C or U), respectively. Each tRNA has unusual secondary structure; tRNA(Ser)GCU lacks the entire D arm, while tRNA(Ser)UGA has a slightly varied cloverleaf structure with an extended anticodon stem. The translation activity of these tRNAs(Ser) was examined by using their transcripts produced by T7 RNA polymerase and an in vitro mitochondrial translation system, showing that tRNA(Ser)GCU has much lower translation ability than that of tRNA(Ser)UGA. HPLC analysis of the product suggested that a certain rate limiting step should exist in the elongation cycle of translation with tRNA(Ser)GCU.