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The maize locus, Opaque‐2, controls the expression in developing endosperm of structural genes encoding a family of storage proteins, the 22 kd zeins, and an abundant albumin, termed b‐32. It is shown that the promoter of the b‐32 gene is activated in vivo in the presence of the O2 gene product and that the information necessary for this activation resides in a 440 bp DNA fragment containing five O2 binding sites (GATGAPyPuTGPu). Two of these sites are embedded in copies of the ‘endosperm box’, a motif thought to be involved in endosperm‐specific expression, which is also represented in 22 kd zein promoters. The O2 protein is also shown to be capable of binding in vitro and activating in vivo, its own promoter.

In this study, evidence is given that a number of isolated coupled plant mitochondria (from durum wheat, bread wheat, spelt, rye, barley, potato, and spinach) can take up externally added K ؉ ions. This was observed by following mitochondrial swelling in isotonic KCl solutions and was confirmed by a novel method in which the membrane potential decrease due to externally added K ؉ is measured fluorimetrically by using safranine. A detailed investigation of K ؉ uptake by durum wheat mitochondria shows hyperbolic dependence on the ion concentration and specificity. K ؉ uptake electrogenicity and the non-competitive inhibition due to either ATP or NADH are also shown. In the whole, the experimental findings reported in this paper demonstrate the existence of the mitochondrial K ؉ ATP channel in plants (PmitoK ATP ). Interestingly, Mg 2؉ and glyburide, which can inhibit mammalian K ؉ channel, have no effect on PmitoK ATP . In the presence of the superoxide anion producing system (xanthine plus xanthine oxidase), PmitoK ATP activation was found. Moreover, an inverse relationship was found between channel activity and mitochondrial superoxide anion formation, as measured via epinephrine photometric assay. These findings strongly suggest that mitochondrial K ؉ uptake could be involved in plant defense mechanism against oxidative stress due to reactive oxygen species generation.One of most outstanding problems in mitochondria bioenergetics concerns the mitochondrial permeability to metabolites, organic compounds, including vitamins, their derived cofactors, and metal ions. The mitochondrial inner membrane contains metabolite carriers (for review, see Refs. 1 and 2), responsible for shuttling substrates between matrix and cytosol and for catabolism dependent on matrix enzymes, as well as vitamin and cofactor translocators (for review, see Refs. 3 and 4). Moreover, the inner membrane also contains the cation carriers and channels that regulate cell and mitochondrial physiology. In particular, as regards K ϩ ion, in mammalian mitochondria, the transport properties are such that net potassium flux across the mitochondrial membrane determines mitochondrial volume (Refs. 5 and 6 and references therein). It has been shown that K ϩ uptake is mediated by diffusion leak, driven by the high electric membrane potential maintained by redox-driven electrophoretic proton ejection, and that regulated K ϩ efflux is mediated by the inner membrane K ϩ /H ϩ antiporter (see Ref. 7). There is also evidence for the existence of an inner membrane protein designed to catalyze electrophoretic K ϩ uptake into mammalian (5-12) and yeast (13, 14) mitochondria. As far as plant mitochondria are concerned, even though mitochondrial structure and function are expected to be strictly dependent on K ϩ transport across the mitochondrial membrane, the knowledge of K ϩ permeability is not established at present. Indeed, the presence of a powerful K ϩ /H ϩ antiporter, which partially collapses ⌬pH, thereby increasing ⌬⌿, 1 has been shown (Refs. 15 and 16 and ...

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