Phagemids are prepared artificially by combining features of phages with plasmids as the name suggests. One such phagemid, which is commonly used in molecular biology laboratories, is “Blue script II KS”, which is derived from PUC19, and is 2961 bp long. The KS designation indicates the “Orientation of poly linkers”, such that the transcription of lac Z gene precedes from the restriction site for Kpn I to that for Sac I.
It may be noted that it has the following features:
- A multiple cloning site (MCS) flanked by T3 and T7 promoters to be read in opposite directions on the two strands.
- An inducible Lac promoter (Lac I), upstream of Lac Z region, which complements with E.coli (lac Z) and provides the facilities for selection of chimeric vector DNA (recombinant DNA) using the criterion of white colonies (as against blue colonies obtained if no foreign DNA is inserted).
- f(+) and f(-) origins of replication derived from a filamentous phage for recovery of sense (+) and anti sense (-) strands of lac Z gene, when host is coinfected with a helper phage.
- An origin of replication (Col EI ori) derived from plasmid, and used in the absence of helper phage.
- A gene for amphicillin resistance for antibiotic selection of chimeric vector.
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