| β-D-galactosidase (β-D-galactoside galactohydrolase, EC3.2.1.23) hydrolyzes the dlsaccharide lactose to glucose and galactose, also referred to as lactase. It is widely distributedin various microorganisms such as bacteria, fungi, and also in various Plants and animals'intestine. Lactase can be used for the treatment of'lactose intolerance', and the Production of low-lactose milk, whey and other dairy Products. Lactase study has important theoretical and practical value. In this study, novel genes encoding lactase with advantage properties were cloned and expressed from microorganism strains isolated from special enviroments, provide material for the further industrial production.We have isolated a strain Dm from soil of The Flaming Mountain. The phylogenetic analysis of the 16s ribosomal DNA sequence from this strain resulted in that it can be classed in the genus cellulomonas sp. .A PCR approach including degenerate PCR, thermal asymmetric interlaced (TAIL) PCR and IPCR we used to clone lactase gene galc. Sequences analysis reveals that the lactase gene with the length of 2076bp encoding a polypeptide of 692 amino acid residues. (G+C) % is 72.9%, and no signal peptide. Structure prediction indicated that galc belonged to the glycosyl hydrolase 42 family, and its protein GALC, have three domains. Use BLAST, the deduced amino acid sequence of this lactase showed 40～60% identities to the previously reported genes,and the highest is to Arthrobacter sp. FB24, is 59%. Suggesting the novelty of this lactase, and it is first reported from Cellulomonas sp.The lactase gene galc was expressed in Escherichia coli BL21 (DE3) and Pichia pastoris GS115 and the properties of prokaryotic expression lactase was purified and characterized. The purfied recombinant lactase had a molecular mass of 76kD. The optimum pH and temperature were 6.4 and 47℃, respectively. At 47℃and pH6.4, the Km was 19.33mmol/L and the Vmax was 416.67nmol/min. Specific activity of galc was about 188.2954 u/mg.
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