Separate sites for binding and nicking of bacteriophage lambda DNA by terminase

The cohesive end site (cos) is the site of action of bacteriophage lambda terminase, the enzyme that introduces staggered nicks to generate the 12-base cohesive ends of mature lambda DNA. Deletion mutations that remove the lambda cohesive end sequence have been isolated after in vitro mutagenesis. The deletions were obtained by digesting the DNA of a cos duplication phage with S1 nuclease to remove the cohesive ends and adjacent base pairs, followed by blunt end ligation and DNA packaging into phage particles. cos2 is the result of a 22-base-pair deletion that exactly removes the segment of rotational symmetry that includes the cohesive end sequence. The cos2 mutation abolishes nicking by terminase but does not affect terminase binding. We conclude that cos contains two sites that interact with terminase: cosN, the nicking site; and cosB, a binding site for terminase.