The Arabidopsis PPDK gene is transcribed from two promoters to produce differentially expressed transcripts responsible for cytosolic and plastidic proteins
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Pyruvate orthophosphate dikinase (PPDK) is a critical enzyme for C4 photosynthesis, providing the primary acceptor for fixation of bicarbonate in mesophyll cells. Although first isolated in C4 plants, it is also present in C3 species. We report that the single gene encoding PPDK in Arabidopsis thaliana possesses two promoters, giving rise to two types of transcript. The longer transcript is generated from a promoter upstream of the first exon, while the shorter transcript is derived from a promoter found within the first intron of the longer form. Apart from 5′ untranslated regions, the presence of the first exon, and three missing codons at the start of the second exon in the longer form, the transcripts are identical. Fusions between the two forms of transcript and gfp showed that the longer transcript encodes a protein targeted to the chloroplast, that its first exon acts as a transit peptide, and that the smaller protein is cytosolic. Abundance of the shorter transcript, responsible for producing the cytosolic protein increases rapidly and specifically during extended dark and dark-induced senescence. Transcripts for both chloroplastic and cytosolic proteins were detectable in cotyledons, while in cauline leaves the transcript encoding the chloroplastic protein was most abundant. We propose that in cotyledons PPDK may be important in supplying PEP to gluconeogenesis, and in ageing leaves it allows remobilisation of nitrogen to supply reproductive tissue.
KeywordsPyruvate orthophosphate dikinase C4 photosynthesis Arabidopsis Nitrogen remobilisation Evolution
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We thank Richard Leegood, John Gray, Chris Chastain and an anonymous referee for useful advice, and Susan Stanley, Naomi Brown, Elisabeth Truernit, Senthil Natesan, Christine Newell and Jesus Badillo Corona for help in the lab. We thank BBSRC for funding (P18931).
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