Structural Characteristics of ACC Synthase Isozymes and Differential Expression of their Genes

Abstract

ACC synthase consists of isozymes that are induced by auxin, ripening or wounding. The expression of each gene was specific to stimulus applied to or generated in tissues. Auxin did not induce the expression of the wound-inducible gene, and wounding did not express the auxin-inducible gene. The primary structures deduced from cDNAs for ACC synthases from different sources were compared in their sequence similarity. The overall similarity between two isozymes induced by wounding and auxin in winter squash is only 55%, and that among three isozymes induced by wounding, auxin and ripening in tomato ranged from 54% to 74%. However, eight regions (box 1 to 8) that include the active site of the enzyme are highly conserved among 14 enzymes. Moreover, numbers of amino acid residues between adjacent regions are constant. Sequential deletion of amino acid residues from the C-terminus of the wound-induced enzyme of winter squash increased the relative specific activity 4-fold, when 56 amino acid residues were deleted, but deletion of 65 residues (deletion of the next 9 residues) resulted in a complete loss of activity. Possible function of these regions in ACC synthase is discussed.