NADP-dependent malic enzyme (NADP-ME, EC 1.1.1.40) catalyzes the oxidative decarboxylation of malate to pyruvate, producing CO, and NADPH. We have examined regulatory properties of a 2.8-kb promoter-leader fragment of a bean (Pbaseolus vurgaris 1.) NADP-ME gene (PvME1) predicted to encode a cytosolic form of the enzyme by expression analysis of promoter-P-glucuronidase fusions in transgenic tobacco plants. l h e PvMEl promoter directed strong expression in stems, which was confined to vascular and pith tissues, and was also active in floral and reproductive tissues. Wounding caused a marked induction of promoter activity, which was further strongly enhanced upon application of stimuli related to pathogen defense. Clutathione (reduced form) was the strongest inducer, but oxidized glutathione, funga1 elicitor, cellulase, catalase, ascorbic acid, and NADPH were additional potent promoterstimulating agents. Responsiveness to reduced glutathione was also shown at the leve1 of PvMEl mRNA accumulation in bean plants. l h e putative contributions of NADP-ME gene expression to the plant defense response and possible mechanisms of defense gene regulation by conditions of oxidative stress as well as by H,O, and antioxidant levels are discussed.
The properties of y-aminobutyric acid (GABA) transport into membrane vesicles derived from synaptosomes of rat brain have been studied using membrane-permeable and -impermeable sulfhydryl reagents, dithiol-specific reagents and oxidizing reagents. GABA transport is inhibited, reversibly, by very low concentrations of the membrane-permeable trivalent arsenical, phenylarsine oxide. Preincubation with this reagent only partially protects GABA transport from inactivation by N-ethylmaleimide (NEM). Thorin, a negatively charged trivalent arsenical, has no influence on GABA transport at concentrations 100-fold higher than that of the inhibitory phenylarsine oxide. The impermeant oxidizing agent, potassium ferricyahide, did not inhibit transport whereas the permeant reagent, diamide, was inhibitory. These data indicate that the GABA transporter possesses an activity-linked dithiol in a hydrophobic region of the carrier not accessible to charged, polar reagents, p-Chloromercuribenzenesulfonate (PCMBS) also inhibits but does not protect against NEM inactivation, suggesting the occurrence of an activity-linked monothiol in a polar region of the carrier.
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