Rice Genetics IV - IRRI books - International Rice Research Institute

(Imaizumi et al 1997). However, the activities of C 4 enzymes such as PPDK are muchlower in C 3 plants than in C 4 plants. It is thus important to identify C 4 -type genes withhigh-level expression.Glackin and Grula (1990) and Sheen (1991) found that the C 4 -type PPDK gene inmaize is transcribed from two different initiation sites under the control of two differentpromoters; the first promoter produces the larger transcript encoding the C 4 -typeenzyme with the chloroplast transit peptide and the second promoter produces the smallertranscript encoding the C 3 -type enzyme that is localized in the cytoplasm. Thelarger transcript is started from exon 1 by the first promoter, whereas the smaller transcriptis started from exon 2 by the second promoter (Fig. 2). We isolated and characterizedthe PPDK gene homologous to the maize C 4 -type gene from rice.The deduced amino acid sequence of rice PPDK is 88% homologous to the maizeC 4 -type PPDK in the mature peptide region (Imaizumi et al 1997). The rice gene issimilar to that of maize except that it has two extra introns (Fig. 2).The transcription activity of the promoters of the rice and maize PPDK was studied.In transgenic rice, both rice and maize promoters directed GUS expression in photosyntheticorgans in a light-dependent manner. However, the maize promoter exhibited amuch higher transcriptional activity than the rice promoter did (Matsuoka et al 1993,Nomura et al 2000). The high-level expression of C 4 -specific genes in rice is not limitedto the PPDK gene, but another C 4 photosynthetic gene for PEPC was also expressedin mesophyll cells in light-grown tissue (Matsuoka et al 1994). These results indicatethat rice contains trans-acting elements that are required for expression of both the C 4PPDK gene and C 4 PEPC gene. It is possible that rice contains most of the trans-actingelements required for the expression of C 4 -specific photosynthetic genes.Rice11’12–50 kb2 33’ 4 5 6 7 8 9 101112 13 14 15 16 1718 19ATGATGTGA1’Maize 6 kb2 34 5 6 7 8 9 10 1112 13141516 1718 19ATG ATG TGA1 kbFig. 2. Exon-intron structure of the genes for PPDK. In both rice and maize genes, transcriptionstarts at the two sites indicated by curved arrows. The common coding regions of the large andsmall transcripts are represented by filled boxes and the 5’ and 3’ noncoding regions are representedby open boxes. Dotted boxes in rice exon 2 and maize exon 1 represent the region thatencodes the transit peptide, and dotted boxes in rice exon 3 and maize exon 2 represent thecoding region unique to the small transcripts. The positions of the initiation and terminationcodons are indicated by ATG and TGA, respectively. The broken lines linking the first and seconddiagrams indicate the presence of two extra introns in the rice gene. (From Matsuoka et al2000.)High-level expression of C 4photosynthetic genes in transgenic rice 441