Regulation of ascorbate oxidase expression by auxin and cell growth in plants.

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Ascorbate oxidase, which catalizes the one-electron oxidation of ascorbic acid to dehydroascorbic acid, occurs in higher plants such as pumpkin and cucumber. The cDNA clone for ascorbate oxidase was isolated from a cDNA library constructed from poly (A) -rich RNA of tobacco cells by screening with pumpkin ascorbate oxidase cDNA.The amino acid sequence deduced from the nucleotide sequence had 68% and 67% identities to those from pumpkin and cucumber ascorbate oxidase cDNA,respectively. These identities were much lower than the identity (80%) between pumpkin and cucumber. The sequence lacks two cysteine residues forming a disulphide bridge, which is necessary to maintain tertiary structure of ascorbate oxidase protein from pumpkin and cucumber. Ascorbate oxidase is significantly expressed only in cucurbitaceous plants. Indeed, ascorbate oxidase activity was slightly detected in tobacco cells and was much lower than that in pumpkin cells. We suggested that ascorbate oxidase is induced by auxin and that the enzyme plays roles for cell prowth and division. We isolated the pumpkin AOBP cDNA,which encodes a protein that binds to an A/T-rich sequence in the promoter of ascorbate oxidase gene. Part of the AOBP protein, which is composed of 52 amino acid residues, is homologous to the Dof domain of a maize DNA-binding protein, MNB1a. The Dof sequences have five conserved Cys residues and are related to the zinc/DNA-binding domains of steroid hormone receptors and GATA1, suggesting that AOBP is a new zinc finger protein.