Construction of plasmids that produce phage P22 repressor.

In a series of plasmid constructions, the c2 (repressor) gene of phage P22 was cloned in a multicopy plasmid and expressed at increasing level. The final result of these constructions is a plasmid that maintains a level of approx. 200 times as much repressor as is found in a lysogen. A series of increasingly virulent phage mutants was isolated by plating sequentially on host cells with increasing levels of repressor. The methods used in the constructions should be applicable to obtaining elevated expression of cloned genes in other systems.