The large subunit of bacteriophage lambda's terminase plays a role in DNA translocation and packaging termination

The DNA packaging enzyme of bacteriophage lambda, terminase, is a heteromultimer composed of a small subunit, gpNu1, and a large subunit, gpA, products of the Nu1 and A genes, respectively. The role of terminase in the initial stages of packaging involving the site-specific binding and cutting of the DNA has been well characterized. While it is believed that terminase plays an active role in later post-cleavage stages of packaging, such as the translocation of DNA into the head shell, this has not been demonstrated. Accordingly, we undertook a generalized mutagenesis of lambda's A gene and found ten lethal mutations, nine of which cause post-cleavage packaging defects. All were located in the amino-terminal two-thirds of gpA, separate from the carboxy-terminal region where mutations affecting the protein's endonuclease activity have been found. The mutants fall into five groups according to their packaging phenotypes: (1) two mutants package part of the lambda chromosome, (2) one mutant packages the entire chromosome, but very slowly compared to wild-type, (3) two mutants do not package any DNA, (4) four mutants, though inviable, package the entire lambda chromosome, and (5) one mutant may be defective in both early and late stages of DNA packaging. These results indicate that gpA is actively involved in late stages of packaging, including DNA translocation, and that this enzyme contains separate functional domains for its early and late packaging activities.