Studies on maize beta glucosidase gene-enzyme system

Maize ß-glucosidase is implicated in phytohormone catabolism, disease resistance mechanisms, and also the catabolism of various ß-D-glucosides. The enzyme expressed in maize sporophytes is encoded by a highly polymorphic locus, Glul (chromosome 10). In the present study, maize ß-glucosidase was purified to homogeneity by using differential solubility and chromatography. The enzyme is soluble and synthesized adaptively after germination. The isoelectric point (pl) of the native enzyme is 4.9-5.0 and its temperature and pH optima are 40°C and 6.8, respectively. The active enzyme is temperature·sensitive and composed of two identical, non-covalently associated and catalytically inactive polypeptides (60 kD each). Enzyme catalysis shows dominant aryl ß-glucosidase and ß-fucosidase activities compared to cellobiase activity. Activity is (1) influenced by the configuration of the C-4 and C-6 atoms on the glycone moeity and by the substrate chain length, (2) possibly mediated by an imidazole ring and a terminal α-amino group in the enzyme catalytic and binding sites, respectively, (3) dependent on intra-chain disulfide bonds to maintain the enzyme conformation, and (4) inhibited competitively by the end product, glucose. The sporophytic specificity of Glul expression might be controlled by tightly linked cis- and trans- acting regulatory elements. One of the several null mutations, with an apparent allelism to Glul locus, (l) complements in trans when combined with normal Glul alleles, and (2) probably affects a shift in the tissue-specific expression of Glul locus (from sporophytic to gametophytic). Another structural gene, GIu3, encoding a soluble, sporophyte-specific, and electrophoretically-invariant ß-glucosidase isoenzyme is present based on hydrodynamic properties, size, surface net charge, peptide map, quaternary structure, and enzyme kinetics.