Phagemid cloning vectors are used widely in biotechnology. These are prepared artificially by combining the features of filamentous phage, M13 with plasmids. A phagemids is a hybrid of a plasmid and a filamentous coliphage that can be propagated in either form.
pBluescript is an example of most commonly used phagemid vector (Figure 14). The pBLUESCRIPT phagemids have both Col E origin and the filamentous phage f1 origin.
Phagemids have following features:
- They contain origin of replication (ori) for double stranded replication and can also be used in the absence of helper phage, as well as an f1 ori to enable single stranded replication and packaging into phage particles.
- A multiple cloning site (MCS) flanked by T3 and T7 promoters to be read in opposite directions on two strands.
- An inducible lac promoter (lacI), upstream of lacZ region, which is complimentary to E. coli (lacZ-) and provides the facility for selection of chimeric vector DNA using the white colonies (as against blue colonies obtained if no foreign DNA is inserted).
- A gene having ampicillin resistance for antibiotic selection.