Poster: Transcription Regulation

Abs # 1033: The use of multiple transcription starts causes the dual targeting of Arabidopsis monodehydroascorbate reductase to mitochondria and chloroplasts

Presenter:	Obara, Keisuke , ss17196@mail.ecc.u-tokyo.ac.jp
Authors	Obara, Keisuke  (A)   Sumi, Kazuyoshi  (B)   Iwamoto, Kuninori  (A)   Fukuda, Hiroo  (A) (C)
Affiliations:	(A): Dept. Biol. Sci., Grad. Sch. Sci., Univ. Tokyo (B): Dept. Biol. Sci., Fac. Sci., Univ. Tokyo (C): Plant Sci. Cent., RIKEN
Web Site:	http://www.biol.s.u-tokyo.ac.jp/users/seigyo/lab.html

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 a-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. Mitochondria and chloroplasts produce a large amount of reactive oxygen species (ROS) by respiration and photosynthesis, respectively. Therefore, the alternative sorting of MDAR proteins to mitochondria or chloroplasts by the alternative use of transcription starts may be regulated according to the activity of respiration and photosynthesis. To examine this possibility we performed semi-quantitative RT-PCR analyses against cDNAs from photosynthetic and non-photosynthetic tissues. We also examined the effect of light on induction of chloroplast-type MDAR. Based on these experiments, we will discuss on the regulation of switching of the transcription starts and alternative targeting of MDAR. Ref) Obara et al. (2002) Plant Cell Physiol. 43: 697-705.