In vitro packaging of a λ Dam vector containing EcoRI DNA fragments of Escherichia coli and phage P1

In this report we describe a coliphage λ vector system for cloning endo R. EcoRI DNA fragments. This system differs significantly from those previously described in two ways. First, restricted and ligated DNA is encapsidated in vitro. Second, with increasing λ DNA size in the range 78 to 100% that of wild-type, the efficiency of DNA encapsidation into infectious phage particles markedly increases. For λ wild-type DNA the efficiency of in vitro packaging (10⁶ to 10⁷ plaques produced per μg of added DNA) is equal to, or better than, the standard CaCl₂ transfection method. The use of a Dam mutation to facilitate recognition of size classes of inserted fragments is described. Using this vector and in vitro packaging, several E. coli and phage P1 endo R.EcoRI fragments were cloned.