pBluescript is an example of a combination between plasmids and phages, thereby gaining it a place within the catergory of vectors known as phagemids. Phagemids represent a hibrid type of class of vectors that serve to produce single-stranded DNA. pBluescript is a popular variety of phagemid commercially available to molecular biology labs for cloning purposes. Through the following descriptions of some key parts of pBluescript, characteristics of plasmids and phages (as discussed above) should be evident (Weaver 2002).Some useful features of pBluescript: Like pUC vectors, which pBluescript is derived from, there is a multiple cloning site inserted into the LacZ' gene, so clones with inserts can be distinguished as blue versus white cells by staining using X-gal. pBluescript also contains the origin of replication of the single-stranded phage f1, which is related to M13 phage vectors. This translates into that a cell harboring a recombinant phagemid, if infected by f1 helper phage that supplies the single-stranded phage DNA replication components, it will produce and package single-stranded phagemid DNA. A final feature of this class of phagemid vectors stems from the fact that it is flanked by two different phage RNA polymerase promoters. More specifically, pBS has a T3 promoter on one end side and a T7 promoter on the other terminal end. This is key because it enables one to isolate the double-stranded phagemid DNA and transcribe it in vitro with either of these two phage polymerases to produce pure RNA transcripts to coincide with either of the two different strands (Brunel 1998).
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