Amber suppression in Escherichia coli by unusual mitochondria-like transfer RNAs.

The "cloverleaf" base-pairing pattern was established as the structural paradigm of active tRNA species some 30 years ago. Nevertheless, this pattern does not accommodate the folding of certain mitochondrial tRNAs. For these recalcitrant tRNAs, we have proposed structures having from 5 to 10 base pairs in the anticodon stem rather than the canonical 6. The absence of these types of tRNAs in cytoplasmic translation systems, however, raises the possibility that they may not be bona fide alternate folding patterns for active tRNA molecules. For this reason, we have designed new tRNA genes based on our model of unusual mitochondrial tRNAs, having 7, 8, 9, and 10 base pairs in the anticodon stem with other modifications to the D-stem and connector regions. We show here that these synthetic genes produce tRNAs that actively suppress amber codons in vivo.