The Gene for Pyruvate,Orthophosphate Dikinase in C4 Plants: Structure, Regulation and Evolution

Pyruvate, orthophosphate dikinase (PPDK; EC 2.7.9.1 EC] ) is a keyenzyme in photosynthesis in plants that exploit the C4 photosyntheticpathway for the fixation of CO₂. This review focuses on thestructure, regulation and evolution of the C4-type ppdk genein the maize genome. The C4-ppdk gene in maize consists of 19exons spanning about 12 kbp. The gene is transcribed from twodifferent initiation sites under the control of two promotersto produce two mRNAs of different sizes. The larger one containsthe exon 1 sequence that encodes the chloroplast transit peptideand its product acts as C4-PPDK in chloroplasts, while the smallerone does not contain the sequence and its product may functionas a C3-enzyme in the cytosol. This unusual dual promoter systemis not unique to the maize C4-type ppdk gene since the sameorganization is also observed in the rice (C3 plant) ppdk geneand in Flaveria. Thus, the two-promoter system is common toplant ppdk genes from C3 and C4, monocot and dicot plants. Adiscussion is also presented of the generation of a system forregulation of the expression of the C4-type ppdk gene. A chimericgene consisting of a reporter gene under the control of thepromoter of maize CA-ppdk is exclusively expressed in photosynthetictissues and not in roots or stems of transgenic rice. The expressionof the introduced gene is also regulated by light: it is lowin etiolated leaves and is enhanced by illumination. These resultsindicate that the regulatory system that controls ppdk expressionin maize is not unique to C4 plants. ¹Recipient of the JSPP Young Investigator Award, 1995.