Gene cloning and characterization of α-glucuronidase of Bacillus stearothermophilus No. 236

The ol-glucuronidase gene of Bacillus stearothermophilus No. 236 was cloned, sequenced, and expressed in Escherichia coli. The gene, designated aguA, encoded a 691-residue polypeptide with calculated molecular weight of 78,156 and pI of 5.34. The alpha -glucuronidase produced by a recombinant E. coli strain containing the aguA gene was purified to apparent homogeneity and characterized. The molecular weight of the or-glucuronidase was 77,000 by SDS-PAGE and 161,000 by gel filtration; the functional form of the alpha -glucuronidase therefore was dimeric. The optimal pH and temperature for the enzyme activity were pH 6.5 and 40 degreesC, respectively. The enzyme's half-life at 50 degreesC was 50 min. The values for the kinetic parameters of K-m and V-max were 0.78 mM and 15.3 U/mg for aldotriouronic acid [2-O-alpha-(4- O-methyl-alpha -D-glucopyranosyluronic)-D-xylobiose]. The or-glucuronidase acted mainly on small substituted xylo-oligomers and did not release methylglucuronic acid from intact xylan. Nevertheless, synergism in the release of xylose from xylan was found when alpha -glucuronidase was added to a mixture of endoxylanase and beta -xylosidase.