Phosphoenolpyruvate carboxylase in mistletoe leaves: Regulation of gene expression, protein content, and covalent modification

Seasonal changes in the activity of phosphoenolpyruvate carboxylase (PEPCas e, EC 4.1.1.31), a key enzyme in the interaction of carbohydrate and nitrog en metabolism, were studied in leaves of the C-3 semiparasitic mistletoe, V iscum album, growing on different host trees. Maximum extractable PEPCase a ctivities were higher in leaves of mistletoes growing on Betula pendula and Alnus glutinosa hosts compared with those on the conifers, Abies alba and Larix decidua Independent of host, maximum extractable PEPCase activities m ere high in spring and autumn while low in summer. Samples with higher PEPC ase activities showed higher amounts of PEPCase protein and higher PEPCase mRNA levels. A curvilinear correlation between leaf total nitrogen content and the maximum extractable PEPCase activity as well as PEPCase mRNA level suggested that nitrogen might affect the activity of PEPCase of mistletoe b y up-regulating gene expression. In addition to extractable activity, seaso nal changes of the PEPCase activation state, the ratio of activities result ing from limited:non-limited assays, were found, which was correlated to th e variation of malate content in leaves of mistletoe, ATP-dependent activat ion of PEPCase was characterized by an increase in I-0.5(L-malate), indicat ing that PEPCase of leaves of mistletoes is probably regulated via phosphor ylation.