The PII protein is a signal integrator involved in the regulation of nitrogen metabolism in bacteria and plants. Upon sensing of cellular carbon and energy availability, PII conveys the signal by interacting with target proteins, thereby modulating their biological activity. Plant PII is located to plastids; therefore, to identify new PII target proteins, PII-affinity chromatography of soluble extracts from Arabidopsis leaf chloroplasts was performed. Several proteins were retained only when Mg-ATP was present in the binding medium and they were specifically released from the resin by application of a 2-oxoglutarate-containing elution buffer. Mass spectroscopy of SDS/PAGE-resolved protein bands identified the biotin carboxyl carrier protein subunits of the plastidial acetyl-CoA carboxylase (ACCase) and three other proteins containing a similar biotin/lipoyl-binding motif as putative PII targets. ACCase is a key enzyme initiating the synthesis of fatty acids in plastids. In in vitro reconstituted assays supplemented with exogenous ATP, recombinant Arabidopsis PII inhibited chloroplastic ACCase activity, and this was completely reversed in the presence of 2-oxoglutarate, pyruvate, or oxaloacetate. The inhibitory effect was PII-dosedependent and appeared to be PII-specific because ACCase activity was not altered in the presence of other tested proteins. PII decreased the V max of the ACCase reaction without altering the K m for acetyl-CoA. These data show that PII function has evolved between bacterial and plant systems to control the carbon metabolism pathway of fatty acid synthesis in plastids.Arabidopsis thaliana | biotin carboxyl carrier protein | PII protein | organic acids | fatty acid metabolism

In higher plants formate dehydrogenase (FDH, EC 1.2.1.2.) is a mitochondrial, NAD-dependent enzyme. We previously reported that in potato (Solanum tuberosum L.) FDH expression is high in tubers but low in green leaves. Here we show that in isolated tuber mitochondria FDH is involved in formate-dependent O 2 uptake coupled to ATP synthesis. The effects of various environmental and chemical factors on FDH expression in leaves were tested using the mitochondrial serine hydroxymethyltransferase as a control. The abundance of FDH transcripts is strongly increased under various stresses, whereas serine hydroxymethyltransferase transcripts decline. The application of formate to leaves strongly enhances FDH expression, suggesting that it might be the signal for FDH induction. Our experiments using glycolytic products suggest that glycolysis may play an important role in formate synthesis in leaves in the dark and during hypoxia, and in tubers. Of particular interest is the dramatic accumulation of FDH transcripts after spraying methanol on leaves, as this compound is known to increase the yields of C 3 plants. In addition, although the steady-state levels of FDH transcript increase very quickly in response to stress, protein accumulation is much slower, but can eventually reach the same levels in leaves as in tubers.FDH catalyzes the oxidation of formate into CO 2 , and is a widespread enzyme in bacteria, yeasts, fungi, mammals, and plants. Several types of FDHs have been reported with differing cofactors (NAD or FAD), electron acceptors, substrates, and cellular locations. A wide diversity of FDH types is found in the bacteria, where they are involved in respiration (for review, see Sawers, 1994) and possibly in the maintenance of a reducing environment (Haynes et al., 1995). The NAD-dependent FDHs (EC 1.2.1.2) have been extensively studied in yeast and bacteria over the past decade because the favorable thermodynamics of the reaction, the easy removal of the product (CO 2 ), and the low cost of the substrate (formate) have made this enzyme a good candidate for industrial NADH regeneration (Allen and Holbrook, 1995). In methanol-utilizing yeasts such as Hansenula polymorpha, Candida boidinii, and Pichia pinus, NAD-dependent FDH plays a crucial role, together with formaldehyde dehydrogenase, in providing NADH to the respiratory chain (Van Dijken et al., 1976). Aside from formate, it is possible that S-formylglutathione might also be a substrate for FDH, as was suggested for yeasts (Van Dijken et al., 1976), pea (Uotila and Koivusalo, 1979), and Pseudomonas sp. 101 (Popov and Lamzin, 1994).In higher plants only NAD-dependent FDHs have been found, and these are localized in the mitochondrial matrix (Halliwell, 1974; Oliver, 1981; Colas des Francs-Small et al., 1993). Oliver (1981) showed that isolated leaf mitochondria from spinach (and, to a lesser extent, that from beet and tobacco) can oxidize formate with a tight coupling to oxidative phosphorylation. The first plant FDH cDNA to be published encodes a potato (So...