The genes involved in C4 photosynthesis have been found in a variety of organisms, not only in C4 (Hatch and Slack 1968) and CAM (Kluge and Osmond 1971) plants but also in C, plants (Imaizumi et al 1997). However, the activities of C4 enzymes such as PPDK are much lower in C, plants than in C4 plants. Therefore, the capability of expressing the C4-type gene for PPDK, Pdk, at high levels is thought to be one of the primary events in establishment of the ability to perform C4 photosynthesis. How did an ancestral gene in C, plants gain a C4-specific mode of expression during the evolution of C4 plants? Glackin and Grula (1990) and Sheen (1991) found that the C4-type Pdk gene in maize is transcribed from two different initiation sites under the control of two different promoters: the first promoter produces the larger transcript encoding the C4-type enzyme with the chloroplast transit peptide and the second promoter produces the smaller transcript encoding the C,-type enzyme that is localized in the cytoplasm. The larger transcript is started from exon 1 by the first promoter, whereas the smaller transcript is started from exon 2 by the second promoter (Fig. 2). To investigate whether this unique mode of expression is also found in the Pdk gene in C, plants, we isolated and characterized a Pdk gene homologous to the maize C4-type gene from a C, plant, rice.
The comparative study revealed that the deduced amino acid sequence of rice PPDK is 88% homologous to the maize C4-type PPDK in the mature peptide region (Imaizumi et al
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