pUC18

pUC18 is a plasmid cloning vector commonly used in Escherichia coli. It belongs to the pUC family of high-copy-number plasmids designed for efficient DNA cloning and easy screening. The vector carries a ColE1-origin of replication, providing a high plasmid copy number in many E. coli strains, and an ampicillin resistance marker (bla) for selection.

A central feature of pUC18 is the lacZ alpha fragment, which enables alpha-complementation-based blue-white screening when used with a compatible host strain and X-gal. The multiple cloning site (MCS) is positioned within the lacZ alpha coding sequence, so the insertion of foreign DNA disrupts lacZ alpha and yields white colonies on indicator plates, whereas intact lacZ alpha results in blue colonies.

The plasmid is relatively small, typically a little over 2.6 kilobases in length, which facilitates straightforward cloning and plasmid preparation. pUC18 and its close relative pUC19 share the same overall design and replication origin, but differ mainly in the content and arrangement of restriction sites within their MCS. This results in slightly different sets of convenient cloning sites and may influence the choice of vector for specific inserts.

Applications for pUC18 include cloning short DNA fragments, library construction, and preparation for sequencing or other analyses. Limitations include a limited insert capacity compared with larger vectors and potential instability of certain inserts in a high-copy context or in some E. coli strains.

In summary, pUC18 is a compact, high-copy cloning vector with an AmpR marker and lacZ-based screening, widely used as a versatile tool in molecular cloning.