Phagemids as vectors

Phagemids combine desirable features of both plasmids and filamentous bacteriophages. They contain (i) origin of replication from plasmid Co1E1, (ii) selectable marker for antibiotic resistance and (iii) major intergenic (IG) region of a filamentous phage, necessary for initiation and termination of viral DNA synthesis and for morphogenesis of phage particles.

Segments of foreign DNA cloned in these vectors are multiplied as in plasmid vectors, but when cells harbouring these plasmids are infected with a suitable filamentous phage (helper phage), the phagemid genome behaves like that of a phage.

The gene II of the infecting helper phage interacts with the intergenic region of phage available in phagemid, so that the phagemid replicates like filamentous phage and generates multiple copies of one strand of the phagemid and the associated foreign DNA inserted in it. These single stranded-copies of the phagemid DNA (including cloned fragment) will be nicked, circularized and packaged into progeny phage particles.