Many plasmid vectors (e.g., the pUC series, Bluescript, pGem, and their derivatives) carry a short segment of Escherichia coli DNA containing the regulatory sequences and the coding information for the first 146 amino acids of β-galactosidase. Vectors of this type are used in host cells that express the carboxy-terminal portion of β-galactosidase. Although neither the host-encoded fragments nor the plasmid-encoded fragments of β-galactosidase are themselves active, they can associate to form an enzymatically active protein. This type of complementation, in which deletion mutants of the operator-proximal segment of the lacZ gene are complemented by β-galactosidase-negative mutants that have the operator-proximal region intact, is called α-complementation. The lac+ bacteria that result from α-complementation are easily recognized because they form blue colonies in the presence of the chromogenic substrate X-Gal. However, insertion of a fragment of foreign DNA into the polycloning site of the plasmid almost invariably results in production of an amino-terminal fragment that is no longer capable of α-complementation. Bacteria carrying recombinant plasmids therefore form white colonies. To screen bacterial colonies, the chromogenic substrate X-Gal and the gratuitous inducer IPTG are mixed with suitable dilution of a culture, combined with molten top agar, and then spread on agar plates containing the appropriate antibiotic. The efficiency of transformation is slightly higher when the bacteria are plated in top agar rather than on the surface of agar plates. Perhaps the transformed bacteria prefer the slightly anaerobic state within the soft agar or the isosmolarity provided by the agar medium.
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