Alpha complementation is a method for bacterial screening that have been transformed with a plasmid vector, carrying the N-terminal coding sequence for β-galactosidase of the lac operon. The lac Z gene is required for galactosidase metabolism. The pUC plasmid carries the alpha fragment of the lac Z gene which is the amino-terminus of the protein and is non functional alone. The omega fragment is found in E. coli chromosome, which is the carboxyterminus of the protein and is nonfunctional alone. When the alpha and omega fragments interact, they become functional and beta-galactosidase protein can be produced. This interaction is known as alpha complementation. It is useful during the screening of beta-galactosidase assay. Artificial galactosides like X-Gal which acts as a substrate for beta-galactosidase and leaves a blue precipitate when hydrolyzed therefore pUC8-transformed E. coli colonies appear blue. They contain plasmids that do not have inserts interrupting the lac Z gene. White colonies consist of bacteria that carry plasmids that have insert-interrupted lac Z genes. (Blue/White Cloning of a DNA Fragment and Assay of β-Galactosidase)
