cosN is the site at which the bacteriophage λ DNA packaging enzyme, terminase, introduces staggered nicks to generate the cohesive ends of mature λ chromosomes. Genetic and molecular studies show that cosN is recognized specifically by terminase and that effects of cosN mutations on λ DNA packaging and cosN cleavage are well correlated. Mutations affecting a particular base-pair of cosN are unusual in being lethal in spite of causing only a moderate defect in cosN cleavage and DNA packaging. The particular base-pair is the rightmost duplex base-pair in mature chromosomes, at position 48,502 in the numbering system of Daniels et al; herein called position - 1. A G·C to T·A transversion mutation at position -1, called cosN-1T, reduces the particle yield of λ fivefold, and the particles formed are not infectious. λcosN-1T particles have wild-type morphology, and contain chromosomes that have normal cohesive ends. The chromosomes of λcosN-1T particles, like the chromosomes of λ+ particles, are associated with the tail. cosN-1T particles, in spite of being normal structurally, are defective in injection of DNA into a host cell. Only approximately 25% of λcosN-1T particles are able to eject DNA from the capsid in contrast to 100% for λ+. Furthermore, for the 25% that do eject, there is a further injection defect because the ejected λ cosN-1T chromosomes fail to cyclize, in contrast to the efficient cyclization found for wild-type chromosomes following injection. The cosN-1T mutation has no effect on Ca2+ mediated transformation by λ DNA, indicating that the effect of the mutation on DNA fate is specific to the process of DNA injection. Models in which specific DNA: protein interactions necessary for DNA injection, and involving the rightmost base-pair of the λ chromosome, are considered. © 1991.
