The opt-matlac sequence was first ligated to plasmid pGAPZaA to generate expres-sion plasmid pGAPZaA-opt-matlac, and verified by sequencing. The recombinant plasmid was linearized by BlnI and transformed to SMD1168H. More than 3,000yeast transformants were obtained on YEPD screening plates, 80 % of which wereblue colonies. A total of 164 transformants with blue radius larger than 5 mm werefurther expressed in a YEPD liquid medium for determination of the enzymaticactivity. The screening identified the strain Lac131 with the highest β-galactosidaseactivity of 504.88 U mL−1