Structure and expression of cDNAs encoding 1-aminocyclopropane-1-carboxylate oxidase homologs isolated from excised mung bean hypocotyls.

By screening a mung bean (Vigna radiata L.) hypocotyl cDNA library using a combination of apple (pAE12) and tomato (pTOM13) 1-aminocyclopropane 1-carboxylate (ACC)-oxidase cDNAs as probes, putative ACC-oxidase clones were isolated. Based on restriction-enzyme map and DNA-sequencing analyses, they can be divided into two homology classes, represented by pVR-ACO1 and pVR-ACO2. While pVR-ACO1 and pVR-ACO2 exhibit close homology in their coding regions, their 3'-noncoding regions are divergent. pVR-ACO1 is a 1312-bp full-length clone and contains a single open reading frame encoding 317 amino acids (MW = 35.8 kDa), while pVR-ACO2 is 1172 bp long and is a partial cDNA clone encoding 308 amino acids. These two deduced amino-acid sequences share 83% identity, and display considerable sequence conservation (73-86%) to other ACC oxidases from various plant species. Northern blot analyses of RNAs isolated from hypocotyl, leaf, and stem tissues using gene-specific probes indicate that the pVR-ACO1 transcript is present in all parts of the seedling and that the expression in hypocotyls is further increased following excision. The maximum induction of ACC-oxidase transcripts occurred at about 6 h after excision, while the maximum enzyme activity was observed at 24 h. When excised hypocotyls were treated with ethylene a further enhanced level of transcripts was observed. Aminooxyacetic acid, an inhibitor of ACC-synthase activity, and 2,5-norbornadiene, an inhibitor of ethylene action, suppressed the wound-induced accumulation of ACC-oxidase mRNA, while an addition of ethylene in these tissues restored the accumulation of ACC-oxidase mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)