Self-association Properties of the Bacteriophage λ Terminase Holoenzyme: Implications for the DNA Packaging Motor Original Research Article

Terminases are enzymes common to complex double-stranded DNA viruses and are required for packaging of viral DNA into a protective capsid. Bacteriophage λ terminase holoenzyme is a hetero-oligomer composed of the A and Nu1 lambda gene products; however, the self-association properties of the holoenzyme have not been investigated systematically. Here, we report the results of sedimentation velocity, sedimentation equilibrium, and gel-filtration experiments studying the self-association properties of the holoenzyme. We find that purified, recombinant λ terminase forms a homogeneous, heterotrimeric structure, consisting of one gpA molecule associated with two gpNu1 molecules (114.2 kDa). We further show that λ terminase adopts a heterogeneous mixture of higher-order structures, with an average molecular mass of 528(±34) kDa. Both the heterotrimer and the higher-order species possess site-specific cos cleavage activity, as well as DNA packaging activity; however, the heterotrimer is dependent upon Escherichia coli integration host factor (IHF) for these activities. Furthermore, the ATPase activity of the higher-order species is ∼1000-fold greater than that of the heterotrimer. These data suggest that IHF bending of the duplex at the cos site in viral DNA promotes the assembly of the heterotrimer into a biologically active, higher-order packaging motor. We propose that a single, higher-order hetero-oligomer of gpA and gpNu1 functions throughout λ development.