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内容記述 |
Alternative splicing has emerged in recent years as a widespread mechanism for regulating gene expression and generating isoform diversity. It has been a subject of major interest, both in its own right as an important biological regulatory mechanism, and also because it has provided insights into some fundamental aspects of splicing. The analysis of alternative splicing has been mainly performed in mammals and some important information about the mechanism such as the identifications of trans factors and cis regulatory elements were obtained. However recent studies demonstrated the involvement of alternative splicing in the synthesis of some enzymes in plants. He analyzed about alternative splicing of two enzymes in pumpkin. One is hydroxypyruvate reductase (HPR) that is known for a leaf peroxisomal enzyme and the other is chloroplastic ascorbate peroxidase (APX).<br /> From the two kinds of cDNA cloning for HPR and the determination of their nucleotide and genomic sequences, it has been proposed that alternative splicing could function in the synthesis of their mRNAs [1]. Therefore, he tried to detect the presence of two HPR proteins in pumpkin cells. The immunofluorescent microscopy using an antibody against HPR showed that HPR1 and HPR2 proteins are localized in leaf peroxisomes and the cytosol, respectively. These localizations were confirmed by the analysis using transgenic plants that expressed fusion proteins with green fluorescent protein. Based on these results and the nucleotide and deduced amino acid sequences, it was concluded that alternative splicing controlled the production of the targeting signal to peroxisomes, resulting in the determination of two HPR proteins. That is, a single HPR gene has two pairs of the GT-AG doublets in the region encoding the carboxy terminus, and the way to splice affects the presence of the targeting signal. The immunoblot analysis revealed that the accumulations of two electrophoretically similar polypeptides corresponding to HPR1 and HPR2 proteins were increased during germination of pumpkin cotyledons and enhanced by light. Moreover the RT-PCR analysis showed that this light induction shifted the splicing pattern from the production of almost equal amounts of HPR1 and HPR2 mRNAs to mainly production of HPR2 mRNA. These results suggest that alternative splicing of HPR pre-mRNA is regulated developmentally and by light [2]. The existence of only HPR2 in stamen tissues in addition to the fact of the much induction of HPR2 protein compared to HPR1 provides the interest of the physiological role of HPR2, but the function of HPR2 protein in the cytosol remains unclear.<br /> To date, cDNAs for HPR in higher plants have been cloned from pumpkin [1] and cucumber [3]. HPR cDNA in cucumber encodes the protein without the targeting signal, namely, the HPR2-type protein. However it was indicated that another HPR protein with the targeting signal to microbodies existed in cucumber because the cucumber HPR gene also has two pairs of the GT-AG doublets at the same position of pumpkin HPR gene. It is therefore important to examine whether another plants have the HPR proteins. Five Arabidopsis EST clones homologous to pumpkin HPR have been registered at the Arabidopsis Biological Resource Center (ABRC) at Ohio State University. The determination of their nucleotide sequences encoding the carboxy termini revealed that all EST clones encode the HPR1-type protein. It was confirmed that only one polypeptide was recognized by antibodies against HPR. At the result of the genomic sequence, Arabidopsis HPR gene has one kind of the GT-AG doublet, although the HPR gene exists as a single copy like does pumpkin HPR gene. These results show that alternative splicing does not undergo in Arabidopsis [4]. The immunoblot analysis demonstrated that pumpkin and cucumber, of tested plants, seem to have two kinds of HPR proteins, whereas another plants seem to have one HPR protein, indicating that all plants do not necessarily need the cytosolic HPR.<br /> Plant cells have four kinds of APX. Of these, two APXs are localized in the stroma (sAPX) and on the thylakoid membrane (tAPX) in chloroplasts, and they are considered as the scavengers of hydrogen peroxide. From the partial sequence at the amino termini and the biochemical properties, it was speculated in their laboratory that sAPX and tAPX might be produced by alternative splicing. Therefore, he tried to isolate sAPX cDNA clone in pumpkin and determine its nucleotide sequence. As compared with the sequence of pumpkin tAPX cDNA [5], sAPX cDNA showed the complete identity except the region encoding the carboxy domain that contained the thylakoid membrane spanning region. The analysis of genomic structure clarified the presence of one donor site and two acceptor sites, indicating that the way to use each acceptor site determined the inclusion of the thylakoid membrane spanning region. The immunobot analysis revealed that this alternative splicing are also regulated developmentally and by light. Moreover, the regulation of tissue-specific manner is present [6].<br /> At the results of these two examples, it was demonstrated that alternative splicing which produces variants whose subcellular or suborganellar localizations are different exists in higher plants. Interestingly, this alternative splicing showed the light-dependency and the tissue-specific manner. Tissue specificity is the phenomena as same as seen in alternative splicing in mammals. However, the light-dependency is specific to plants, indicating the presence of the novel mechanism of the signal transduction. Also, these data suggest that the conversions of microbodies (from glyoxysomes to leaf peroxisomes) and chloroplasts (from etioplasts to chloroplasts) are closely related to the regulation of HPR and chloroplastic APXs by alternative splicing, respectively.<br /><br />[1] Hayashi M., Tsugeki R., Kondo M., Mori H and Nishimura M. (1996) Plant Mol. Biol., 30, 183-189.<br />[2] Mano S., Hayashi M. and Nishimura M. (1998) Plant Cell, (submitted)<br />[3] Greenler J. M., Sloan J. S., Schwartz B. W. and Becker W. M. (1989) Plant Mol. Biol., 13, 139-150.<br />[4] Mano S., Hayashi M., Kondo M. and Nishimura M. (1997) Plant Cell Physiol. 38, 449-455.<br />[5] Yamaguchi K., Hayashi M. and Nishimura M. (1996) Plant Cell Physiol. 37, 405-409.<br />[6] Mano S., Yamaguchi K., Hayashi M. and Nishimura M. (1997) FEBS Lett. 413, 21-26. |
要旨・審査要旨 / Abstract, Screening Result (354.0 kB)
