Cloning and expression of the β-galactosidase genes from Lactobacillus reuteri in Escherichia coli

Heterodimeric β-galactosidase of Lactobacillus reuteri L103 is encoded by two overlapping genes, lacL and lacM. The lacL (1887 bp) and lacM (960 bp) genes encode polypeptides with calculated molecular masses of 73,620 and 35,682 Da, respectively. The deduced amino acid sequences of lacL and lacM show significant identity with the sequences of β-galactosidases from other lactobacilli and Escherichia coli. The coding regions of the lacLM genes were cloned and successfully overexpressed in E. coli using an expression system based on the T7 RNA polymerase promoter. Expression of lacL alone and coexpression of lacL and lacM as well as activity staining of both native and recombinant β-galactosidases suggested a translational coupling between lacL and lacM, indicating that the formation of a functional β-galactosidase requires both genes. Recombinant β-galactosidase was purified to apparent homogeneity, characterized and compared with the native β-galactosidase from L. reuteri L103.