Phasmids, Plasmid Replication Functions, Lambda Repressor, El Protein, Molecular Hybridization

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	Phasmids - Plasmids are truly plasmids with phage genes. They are linear duplex DNAs whose ends are λ segments that contain all the genes required for a lytic infection and whose middle is linearized. Both the λ and the plasmid replication functions are intact. Normally, plasmid vectors carry a λ attachment site. Once inside the E. coli cell, the plasmid can replicate like a phage and form plaques in the normal way.
However, if the vector contains the gene that encodes λ repressor, then the plasmid replicates as a plasmid rather than as a phage. Depending upon the functioning or non functioning of el-Protein (coded by λ-repressor), the plasmid can replicate as plasmid (cl-Protein inactive) or phage when cl-protein is active. The activity of el-protein can be inactive by growing the E.coli culture at 40°C Plasmids may be used in variety of ways. For example, DNA may be cloned in the plasmid vector in a conventional way and then the recombinant plasmid can be lifted onto the phage. Plasmids are easy to store, they have an effectively infinite shelf life and screening phages by molecular hybridization gives cleaner results than screening bacterial colonies.