Figure 1. Model of lambda genome packaging.

A-Upper: Packaging initiation. The terminase protomer is a tight association of two TerS^λ subunits and one TerL^λ subunits. In the N-terminal domain (NTD) of TerS^λ is a winged helix-turn-helix DNA binding motif and the TerS^λ dimerization interface. The TerS^λ C-terminal domain (CTD) includes a functional domain for interacting with TerL^λ’s N-terminus. TerL^λ consists of two globular domains connected by a linker. The TerL^λ NTD contains the DNA packaging ATPase and the CTD contains the maturation endonuclease. Four protomers form into a ring-like structure at the cos site of a lambda genome concatemer, assembling the maturation complex. Maturation complex assembly involves interactions of TerS^λ with cosB, and the TerL^λ endonuclease with cosN, and requires ATP and/or IHF. Following duplex nicking by TerL^λ and ejection of the upstream, cosQ-containing DNA end yields the post-cleavage complex (Complex I). Complex I docks at the portal of an empty procapsid, producing the packaging motor complex. Complex I docking involves an interaction between the TerL^λ C-terminal tether and the portal protein. Docking activates the packaging ATPase, which powers translocation of DNA into the procapsid shell. We note that the stoichiometry of the protomer subunits bound to the procapsid, along with the subunit arrangement in the maturation complex, remain speculative at this time. A-Lower: Translocation and virion assembly. Progression of translocation is shown proceeding left to right. When ~30% of the DNA has been translocated, procapsid expansion is triggered, and translocation proceeds until the next cos along the concatemer is encountered. Interactions between the translocating terminase and cosQ and cosN lead to nicking of the downstream cos, and terminase undocking. Addition of gpW and gpFII to the portal is followed by tail attachment, generating a mature virion. B. ATPase motif sequences. Walker B motif and catalytic glutamate ASCE sequences are given for TerLs discussed in this paper. Beginning and ending residue numbers are given in superscripts. *Note that TerL^Sf6 contains a deviant glutamate (red) rather than the canonical aspartate found in most ASCE Walker B sequences