The lacZ gene is a gene present in E. coli that encodes the protein beta-galactosidase. Beta-galactosidase is an enzyme that is essential for the breakdown of lactose as it cleaves a bond between the two carbon rings in lactose to produce glucose and galactose.
LacZ is a part of the gene family called the lac operon. The lac operon is a system of three genes along with a promoter, terminator, operator and regulator. The three genes present in the lac operon are lacZ, lacY and lacA. The other two genes in the lac operon, lacY and lacA, are also used in the breakdown of lactose but do not have the same function as lacZ. Only the lacZ product, beta-galactosidase, can break the bond between galactose and glucose in lactose molecules.
LacZ is an important gene in the study of E.coli because it is used as a reporter gene in bacteria that do not normally express it. In this case, lacZ is introduced into the bacteria, typically as a plasmid or short circular piece of DNA. When bacteria that have the lacZ gene introduced into them are grown on a particular type of media called X-gal, only the bacterial colonies expressing lacZ and thus beta-galactosidase are capable of breaking the bond in the X-gal medium to produce galactose and a blue dye. Therefore, the bacteria properly expressing the plasmid of interest appear blue.
This technique is used in many microbiology experiments involving research into the expression of a particular gene or promoter because it allows researchers to quickly determine that the gene of interest is being expressed.