1-Amino-cyclopropane-1-carboxylate synthase (ACS, EC 4.4.1.14) is the key enzyme in the ethylene biosynthetic pathway in plants. The completion of the Arabidopsis genome sequence revealed the presence of twelve putative ACS genes, ACS1-12, dispersed among five chromosomes. ACS1-5 have been previously characterized. However, ACS1 is enzymatically inactive whereas ACS3 is a pseudogene. Complementation analysis with the Escherichia coli aminotransferase mutant DL39 shows that ACS10 and 12 encode aminotransferases. The remaining eight genes are authentic ACS genes and together with ACS1 constitute the Arabidopsis ACS gene family. All genes, except ACS3, are transcriptionally active and differentially expressed during Arabidopsis growth and development. IAA induces all ACS genes, except ACS7 and ACS9; CHX enhances the expression of all functional ACS genes. The ACS genes were expressed in E. coli, purified to homogeneity by affinity chromatography, and biochemically characterized. The gas ethylene has been known since the beginning of the century to be used by plants as a signaling molecule for regulating a variety of developmental processes and stress responses (1). These include seed germination, leaf and flower senescence, fruit ripening, cell elongation, nodulation, wounding and pathogen responses. Ethylene production is induced by a variety of external factors, including wounding, viral infection, elicitors, auxin treatment, and Li ϩ ions (2-7). Ethylene is biosynthesized from methionine, which is converted to AdoMet 1 by the enzyme AdoMet synthetase. AdoMet is converted by the enzyme ACS to ACC, the precursor of ethylene (5, 7-9). ACC is oxidized to ethylene by ACO. ACS is a pyridoxal phosphate-containing enzyme (2, 5, 10) and its activity is regulated at the transcriptional (11-18) and posttranscriptional levels (19 -21). The ethylene biosynthetic enzymes, AdoMet synthetase, ACS, and ACO, are encoded by multigene families in various plant species (5,9,16,17,(22)(23)(24)(25). The crystal structures of apple and tomato Le-ACS2 isozymes were recently elucidated (26, 27). The structures show that the enzyme is a homodimer, and its overall fold and active sites are similar to those of aminotransferases even though the two enzymes have completely different catalytic activities. The tertiary structures together with available biochemical data explain the catalytic roles of the conserved and non-conserved active site residues (28 -31).The sequencing of the Arabidopsis genome revealed that ACS genes are putatively encoded by twelve genes (32). The question immediately arises as to why there are so many ACS isozymes for synthesizing ethylene in Arabidopsis. It has been postulated that the presence of ACS isozymes may reflect tissue-specific expression that satisfies the biochemical environment of the cells or tissues in which each isozyme is expressed (15). For example, a group of cells or tissues with low concentration of the ACS substrate, AdoMet, would express a high affinity (low K m ) ACS isozyme. Accordingly, the ...
