Genomic structure and expression of the pyruvate, orthophosphate dikinase gene of the dicotyledonous C4 plant Flaveria trinervia (Asteraceae)

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Abstract
Pyruvate orthophosphate dikinase (PPDK) is a key enzyme of C4 photosynthesis providing the acceptor molecule for the primary CO2 fixation in the mesophyll cells. Here we present the isolation and characterisation of the corresponding gene (termed pdk) from the C4 plant Flaveria trinervia (Asteraceae). Southern analysis indicates that in contrast to maize pdk sequences in F. trinervia are present as single copy. Sequence analysis of the entire gene reveals that its coding sequence is identical to the previous isolated PPDK-cDNA from this species. The gene spans about 13 kb and consists of 21 exons, it thus contains two additional exons compared to the maize gene. As in maize, a long intervening sequence of 6.1 kb is positioned at the boundary of the transit peptide segment and the mature protein region. Pdk transcripts accumulate abundantly in leaves, but are also detectable in stems and roots. While the leaf and stem transcripts are 3.4 kb in size and encode the chloroplastic PPDK isoform, a 3.0 kb transcript lacking the region encoding the plastidic transit peptide accumulates in roots. Thus two different transcripts can be produced from a single pdk gene most likely by use of alternative promoters and not by alternative splicing. The accumulation of the 3.4 kb transcript is under light control. Darkening leads to a drastic depletion of this transcript in both leaves and stems. Instead, the 3.0 kb transit peptide-lacking pdk transcript accumulates, but only in stems and roots, not in leaves.