Molecular cloning of thermostable β-glucosidase gene from a thermophilic anaerobe NA10 and its high expression in Escherichia coli

A gene encoding β-glucosidase was cloned from a thermophilic anaerobe strain NA10 into Escherichia coli. The 4.6 kb of HindIII fragment was shown to direct the synthesis of β-glucosidase. A homologous DNA sequence was found to be present in strain NA10 by Southern blot analysis. The β-glucosidase was purified from the recombinant E. coli. The molecular weight of the enzyme was estimated to be 50.1 kDal using SDS-polyacrylamide gel electrophoresis. The enzyme was heat stable; the optimum pH and temperature were 5.5 and 85°C, respectively. The enzyme could hydrolyse low molecular weight β-1,3 and β-1,4 glucans. The β-glucosidase was produced in the cytosol of the recombinant E. coli (7-20% of total protein), without fusion to any E. coli promoter sequences.