Phosphorylation of phosphoenolpyruvate carboxylase plays a key role in the control of plant metabolism. Phosphoenolpyruvate carboxylase kinase is a Ca2+-independent enzyme that is activated by a process involving protein synthesis in response to a range of signals in different plant tissues. The component whose synthesis is required for activation has not previously been identified, nor has the kinase been characterised at a molecular level. We report the cloning of phosphoenolpyruvate carboxylase kinase from the Crassulacean Acid Metabolism plant Kalanchoë fedtschenkoi and the C3 plant Arabidopsis thaliana. Surprisingly, phosphoenolpyruvate carboxylase kinase is a member of the Ca2+/calmodulin-regulated group of protein kinases. However, it lacks the auto-inhibitory region and EF hands of plant Ca2+-dependent protein kinases, explaining its Ca2+-independence. Its sequence is novel in that it comprises only a protein kinase catalytic domain with no regulatory regions; it appears to be the smallest known protein kinase. In K. fedtschenkoi, the abundance of phosphoenolpyruvate carboxylase kinase transcripts increases during leaf development. The transcript level in mature leaves is very low during the photoperiod, reaches a peak in the middle of the dark period and correlates with kinase activity. It exhibits a circadian oscillation in constant conditions. Protein kinases are typically regulated by second messengers, phosphorylation or protein/protein interactions. Phosphoenolpyruvate carboxylase kinase is an exception to this general rule, being controlled only at the level of expression. In K. fedtschenkoi, its expression is controlled both developmentally and by a circadian oscillator.

Phosphorylation of phosphoenolpyruvate carboxylase (PEPc; EC 4.1.1.31) plays an important role in the control of central metabolism in higher plants. Two PPCK (PEPc kinase) genes have been identified in tomato (Lycopersicon esculentum cv Alicante), hereafter termed LePPCK1 and LePPCK2. The function of the gene products has been confirmed by transcription of full-length cDNAs, translation, and in vitro assay of kinase activity. Previously studied PPCK genes contain a single intron. LePPCK2 also contains a novel second intron that exhibits alternative splicing. The correctly spliced transcript encodes a functional PEPc kinase, whereas unspliced or incorrectly spliced transcripts encode a truncated, inactive protein. The relative abundance of the transcripts depends on tissue and conditions. Expression of LePPCK2 was markedly increased during fruit ripening. In ripe Alicante fruit, the locule and seeds contained only the correctly spliced LePPCK2 transcripts, whereas in ripe fruit of the tomato greenflesh mutant, they contained correctly and incorrectly spliced transcripts. Potato (Solanum tuberosum) contains genes that are very similar to LePPCK1, and LePPCK2; StPPCK2 exhibits alternative splicing. Aubergine (Solanum melongena) and tobacco (Nicotiana tabacum) also contain a PPCK2 gene; the sequence of the alternatively spliced intron is highly conserved between all four species. The data suggest that the two PPCK genes have different roles in tissue-specific regulation of PEPc and that the alternative splicing of PPCK2 transcripts is functionally significant.