The use of multiple transcription starts causes the dual targeting of Arabidopsis putative monodehydroascorbate reductase to both mitochondria and chloroplasts.

Monodehydroascorbate reductase (MDAR) isoforms exist in mitochondria, chloroplasts, cytosol and microbodies. Two putative MDAR sequences with an extended N-terminal region are found in Arabidopsis. They differ in the length of the extension by 21 bp. 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 fusion proteins were imported into mitochondria and chloroplasts, respectively. These results demonstrate that putative MDAR is a dual-targeting protein transported into both mitochondria and chloroplasts. Although there have been several reports of dual targeting of proteins to mitochondria and chloroplasts, this is the first example in which the dual targeting of the protein to mitochondria and chloroplasts is achieved by the use of multiple transcription initiation sites.