Cosmid Packaging and Infection of E. coli

Cosmids are cloning vectors that were developed to enable large fragments of DNA to be cloned and maintained ( 1 – 3 ). Cosmid vectors allow the cloning of fragments up to 45 kilobases (kb) and are commonly used in genomic library construction. The distinguishing feature of a cosmid is the presence of bacteriophage λ cos sites, which enable the vector, and cloned genomic DNA, to be packaged into bacteriophage heads ( 1 , 4 ). Cosmids also contain several other components, including a drug-resistance marker that is necessary for the selection of cosmid-containing bacteria, a plasmid origin of replication, usually ColE1, which regulates cosmid replication and copy number within the Escherichia coli host ( 1 , 5 ), and restriction sites that facilitate cloning of the desired DNA fragments and insert verification by restriction digestion ( see Fig. 1 ).

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