β-galactosidase is an exoglycosidase which hydrolyzes the β-glycosidic bond formed between agalactose and its organic moiety. It may also cleave fucosides and arabinosides but with much lower efficiency. It is an essential enzyme in the human body. Deficiencies in the protein can result ingalactosialidosis or Morquio B syndrome. In E. coli the gene of β-galactosidase the lacZ gene is present as part of the inducible system lac operon which is activated in the presence of lactose when glucose level is low.
It is commonly used in molecular biology as a reporter marker to monitor gene expression. It also exhibits a phenomenon called α-complementation which forms the basis for the blue/white screening of recombinant clones. This enzyme can be split in two peptides LacZα and LacZΩ neither of which is active by itself but when both are present together spontaneously reassemble into a functional enzyme. This property is exploited in many cloning vectors where the presence of the lacZα gene in a plasmid can complement in trans another mutant gene encoding the LacZΩ in specific laboratory strains of E. coli. However when DNA fragments are inserted in the vector the production of LacZα is disrupted the cells therefore show no β-galactosidase activity. The presence or absence of an active β-galactosidase may be detected by X-gal which produces a characteristic blue dye when cleaved by β-galactosidase thereby providing an easy means of distinguishing the presence or absence of cloned product in a plasmid.
E. coli β-Galactosidase consists of 1024 amino acids with molecular mass of 116 kDa and functional form is a homotetramer.