M13 is a single-stranded DNA bacteriophage that infects male Escherichia coli strains. A chronic infection is set up during which the intracellular replicative form of the phage (RF-I, double-stranded DNA) serves as template for single-stranded ([+]strand) DNA production. The single-stranded DNA, packaged in phage coat proteins, is extruded through the host cell wall into the medium in large quantities.
M13mp18/19 are M13 derivatives. Each contains a multiple cloning site (MCS) within a portion of the gene for β-galactosidase. This DNA segment corrects a defect in β-galactosidase production by the host strain. The MCS regions of M13mp18/19 contain 13 unique restriction endonuclease sites, respectively.
Foreign DNA incorporated at the multiple cloning site results in loss of β-galactosidase production and consequently the inability to use lactose as a carbon source. Such lactose negative strains are detected as white plaques on medium containing X-Gal, a chromogenic substrate for β-galactosidase.
The intracellular, double-stranded (RF-I) form of the M13 phage can be manipulated like plasmid DNA, while the extracellular, single-stranded DNA is an excellent source of material for determining the sequence of insert DNA by the dideoxy procedure. The multiple cloning site of M13mp9/19 is in the opposite orientation relative to the cloning site of M13mp8/18.
Unique sites: Within the β-galactosidase gene - Bgl I, Mst II, Pvu I; other unique sites - Aha II, ApaL I, Ava II, Bal I, Ban II, Bgl II, Bsm I, Dra III, HgiD I, Nae I, Nar I, Sau I, Sin I, SnaB I, Sno I.
Note: Attempts to clone foreign DNA at these "other" sites will likely lead to inactivation of essential phage functions.