The Arabidopsis PPDK gene is transcribed from two promoters to produce differentially expressed transcripts responsible for cytosolic and plastidic proteins.
Pyruvate orthophosphate dikinase (PPDK) is a critical enzyme for C(4) photosynthesis, providing the primary acceptor for fixation of bicarbonate in mesophyll cells. Although first isolated in C(4) plants, it is also present in C(3) 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.
Parsley
Kate
K
Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK.
Hibberd
Julian M
JM
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics*; D001483:Base Sequence; D003600:Cytosol / Q000378:metabolism*; D017931:DNA Primers; D018076:DNA, Complementary; D017343:Genes, Plant; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000096:biosynthesis / Q000235:genetics*; D011401:Promoter Regions, Genetic; D011772:Pyruvate, Orthophosphate Dikinase / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic
D017931:DNA Primers; D018076:DNA, Complementary; D010940:Plant Proteins; D012333:RNA, Messenger; D011772:Pyruvate, Orthophosphate Dikinase