Cosmids

Cosmids are conventional vectors that contain a small region of bacteriophage X DNA containing the cohesive end site (cos). This contains all of the cis-acting ele­ments for packaging of viral DNA into X particles. For cloning of DNA in these vec­

Table 1

Commonly Used Cloning Vectors
Plasmid	Features	Commercial source
pUC18, pUC19	Small size (2.7 kb) High copy number Multiple cloning site Ampicillin-resistance marker Blue/white selection (see Part 19)	NEB
pBluescript vectors	As pUC Single-stranded replication origin T7 and SP6 promoters flanking MCSa	Stratagene
pACYC vectors	Low copy number (15 copies per cell) p15A origin of replication	NEB
Supercos	Cosmid vector Two cos sites Insert size 30-42 kb Ampicillin-selectable marker T3 and T7 promoters flanking cloning site	Stratagene
EMBL3	X replacement vector MCS sites: Sail, BamHI and EcoRI	Promega
X ZAP	X vector In vivo excision into pBluescript phagemid vector Cloning capacity 10 kb Blue/white selection	Stratagene
pBeloBAC11	BAC vector Inserts up to 1 Mb T7 and SP6 promoters flank insertion site Blue/white selection Cos site LoxP site	NEB

tors, linear genomic DNA fragments are ligated in vitro to the vector DNA and this is then packaged into bacteriophage particles (see Part 7). On introduction into Escherichia coli host cells, the vector is circularized to form a large plasmid contain­ing the cloned DNA fragment. Cosmids are most commonly used to generate large insert libraries. Because of the constraints of packaging, the vector plus insert should comprise between 28 and 45 kb.