Structure of the bacteriophage lambda cohesive end site. Genetic analysis of the site (cosN) at which nicks are introduced by terminase.

A collection of mutations affecting the site (cosN) at which the bacteriophage lambda DNA packaging enzyme, terminase, introduces nicks to generate mature lambda chromosomes has been studied. A good correlation was found for mutational effects on burst size, accumulation of unused proheads, packaging of DNA into heads and cos cutting by terminase in vitro, indicating that defective cosN cleavage by terminase is the molecular explanation for the phenotypic effects of the mutations. Although the base-pairs of cosN display partial twofold rotational symmetry, cosN was found to be asymmetric functionally. Certain mutations to the left side of the center of rotational symmetry have more pronounced phenotypic effects than rotationally symmetric mutations to the right. The cosN11G mutation has no phenotypic effects when present as a single mutation, but does affect DNA packaging and cosN cutting in the presence of the symmetrically disposed cosN2C mutation. Mutations that decrease cosN cleavage result in the accumulation of unexpanded proheads, indicating that prohead expansion depends on cosN cutting.