Artificial mitochondrial presequences.

Synthetic oligonucleotides were used to construct artificial mitochondrial presequences that contained, besides the initiator methionine, only arginine, serine, and leucine. The ratio of these three amino acids was adjusted to match that of basic, hydroxylated, and hydrophobic residues in natural mitochondrial presequences. When these sequences were fused to the N terminus of yeast cytochrome oxidase subunit IV lacking its own presequence, they directed the attached subunit IV to its correct intramitochondrial location in vivo. They also mediated import of subunit IV into isolated yeast mitochondria. In contrast, artificial sequences containing glutamine, arginine, and serine residues following the initiator methionine were inactive. Thus, the targeting function of mitochondrial presequences does not depend on specific amino acid sequences but may instead depend on the overall balance between basic, hydrophobic, and hydroxylated amino acids.