M13 Phagemids and Rescue

Just as actual M13 phages are useful in recombinant DNA technology, there are plasmids available that have the M13 origin of replication, yet none of the genes for virus production (notably, genes 2, 3, 5, and 8). We refer to these plasmids as phagemids because they can be replicated and packaged by M13 proteins, if the transfected cell is superinfected with M13 helper phage.  This helper phage provides the necessary gene products to replicate and package our phagemids as ssDNA phages.

The particular M13 phagemid we cover is pBluescript, shown above. 

  1. Being a fairly small plasmid, it has high copy number,
  2. The Ampr gene is a selectable marker (cells transformed by the plasmid exhibit ampicillin resistance, and can be selected for)
  3. The presence of the lac-Z-alpha fragment (and the MCS within the fragment) represent the scorable marker; transformed cells that contain unaltered plasmid will produce blue colonies on IPTG/X-gal plates, transformed cells with plasmid that have insert disrupting the lac-Z-alpha fragment will produce white colonies.
  4. The f1 origin allows for phagemid rescue.

Phagemid rescue is accomplished by the superinfection of helper phage, mentioned above (M13K07 in our case). This phage provides the necessary proteins for phage packaging, and we can recover the plasmid in single-stranded form inside phage particles.