A novel beta-galactosidase gene (Bgal) was cloned from the marine bacterium Alteromonas sp. ML117. Bgal from Alteromonas sp. ML117 was heterologously expressed in Escherichia coli BL21 (DE3) cells to study the enzymatic properties of the recombinant beta-galactosidase. Using gel-filtration chromatography and SDS-PAGE, the recombinant enzyme was identified to be a tetrameric enzyme that belonged to the glycoside hydrolase family GH2. The purified recombinant Bgal was able to hydrolyze lactose and o-nitrophenyl-beta-D-galactopyranoside (ONPG), and showed maximum activity at pH 8.0 and 30 degrees C to hydrolyze ONPG and at 35 degrees C to hydrolyze lactose. The enzyme's activity level quickly diminished when incubated at 35 degrees C, suggesting that it was a cold-adapted variant. At 10 degrees C, the purified enzyme had K-m values of 1.6 mM for ONPG and 3.8 mM for lactose. The enzyme tolerated NaCl, and retained 80% of its activity in a 30% ethanol reaction system. In addition, the recombinant Bgal hydrolyzed 86% of the lactose from milk over 24 h at 10 degrees C. Taken together, this study identified a recombinant Alteromonas sp. ML117 beta-galactosidase that may be used for industrial applications.