lacZ

LacZ is a gene in Escherichia coli that encodes the enzyme beta-galactosidase. It is part of the lac operon, which also includes lacY and lacA, and it is commonly used in molecular biology as a reporter gene to monitor gene expression and promoter activity. In its native context, lacZ expression is regulated along with the operon by the LacI repressor and the presence of lactose or its metabolite allolactose, with catabolite repression influenced by glucose through CAP-cAMP.

Beta-galactosidase catalyzes the hydrolysis of beta-galactosides, including lactose, to glucose and galactose. In laboratory settings, substrates such as X-gal and ONPG are used to visualize or quantify activity. X-gal yields a blue precipitate in colonies expressing functional LacZ, enabling blue-white screening to distinguish recombinant clones. ONPG is a soluble substrate that produces a yellow color upon cleavage, allowing quantitative measurement of beta-galactosidase activity.

In cloning and reporter assays, lacZ is frequently used as a reporter gene or as part of alpha-complementation systems. In alpha-complementation, a plasmid carries the lacZ' fragment while the host provides the lacZ omega fragment, reconstituting enzymatic activity and enabling blue/white screening. Mutant or truncated versions of lacZ can affect reporter performance and background activity, necessitating appropriate controls.

LacZ has played a foundational role in genetics and molecular biology, providing a versatile and well-characterized means to study gene expression, promoter strength, and cloning efficiency. See also lac operon, blue-white screening, and beta-galactosidase assays.