Abstract
Monodehydroascorbate reductase (MDAR) isoforms exist in mitochondria, chloroplasts, cytosol and microbodies. Two MDAR amino acid sequences with an extended N-terminal region are found in Arabidopsis. They differ in the length of the extension by 7 amino acid residues. We have shown that these two isoforms arise from a single gene by the use of multiple transcription starts. Green fluorescent protein was fused to each extension, revealing that the longer and shorter isoforms were imported into mitochondria and chloroplasts, respectively. These results demonstrate that MDAR is a dual-targeting protein transported into mitochondria and chloroplasts. The 7 amino acid residues extend in the longer one was predicted to form a potent positively charged amphiphilic α-helix often seen in mitochondrial presequences. Although there have been several reports of dual targeting of proteins to mitochondria and chloroplasts, this is the first example in which it is achieved by the use of multiple transcription starts.
