Abstract
The genome of temperate phage X is a linear duplex DNA molecule with single stranded termini. The single ends contain complementary base sequences allowing X DNA to circularize in vitro. Treatment of such hydrogen bonded circles with DNA ligase produces a covalently-closed circle. Invivo λ DNA replicates via the rolling circle model (1) and the viral DNA is packaged into phage heads as linear molecules. The cohesive ends of λ DNA appears to serve as initiation sites for packaging the λ genome (2). Insertion of the cohesive ends (cos) of λ DNA into a plasmid, by recombinant DNA technology, confers on the resulting cos plasmid susceptibility to packaging by λ invivo or invitro, if the size of the plasmid approximates that of the viral genome (3). Insertion of cos into a small plasmid, such as pBR322, generates a cos plasmid that can only be packaged into mature phage particles if additional DNA is cloned into the small cos plasmid to increase its molecular size. Thus, λ cos plasmids or cosmids serve as excellent vectors for cloning rather large DNA segments (3).