STRUCTURE AND EXPRESSION OF 3 GENES ENCODING ACC OXIDASE HOMOLOGS FROM MELON (CUCUMIS-MELO L)

The enzyme ACC oxidase catalyses the last step of ethylene biosynthesi s in plants, converting 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. We have previously described the isolation and characteriza tion of a cDNA clone (pMEL1) encoding an ACC oxidase homolog from melo n (Cucumis melo L.). Here we report the isolation and characterization of three genomic clones, corresponding to three putative members of t he ACC oxidase gene family in melon. All are transcriptionally active. The sequences of these genes have been determined. One genomic clone (CM-ACO1), corresponding to the cDNA previously isolated, presents a c oding region interrupted by three introns. Its transcription initiatio n site has been defined with RNA from ripe fruit and ethylene-treated leaves. The other two genes (CM-ACO2, CM-ACO3) have only two introns, at positions identical to their counterparts in CM-ACO1. The degree of DNA homology in the coding regions of CM-ACO2 and CM-ACO3 relative to CM-ACO1 is 59% and 75%, respectively. CM-ACO2 and CM-ACO3 are 59% hom ologous in their coding regions. These three genes have close homology to PH-ACO3, a member of the ACC oxidase multigene family of petunia. The predicted amino acid sequences of CM-ACO1 and CM-ACO3 are 77% to 8 1% identical to those encoded by the tomato and petunia genes, while t he deduced amino acid sequence of CM-ACO2 shows only 42% to 45% homolo gy. RT-PCR analysis using gene-specific primers shows that the three g enes are differentially expressed during development, ethylene treatme nt and wounding. CM-ACO1 is induced in ripe fruit and in response to w ounding and to ethylene treatment in leaves. CM-ACO2 is detectable at low level in etiolated hypocotyls. CM-ACO3 is expressed in Rowers and is not induced by any of the stimuli tested.