Figure 1.

Lambda terminase and assembly of a virus particle. (A) The lambda terminase protomer is composed of one catalytic subunit (TerL) and two DNA binding subunits (TerL₁•TerS₂, blue and cyan, respectively). Three functional nucleotide binding sites have been identified in the protomer (see (23) for a detailed discussion); (i) a low-affinity site in TerS that binds ATP, ADP, GTP and GDP (NXP) and that regulates DNA binding interactions. NXP binding to this site also stimulates packaging ATPase activity in TerL; (ii) a high-affinity packaging ATPase site in the N-terminal DNA packaging domain that powers DNA translocation by the packaging motor; (iii) a high-affinity site in the C-terminal genome maturation site in TerL that binds ATP and that is the focus of the present work. (B) Model for genome maturation and packaging by lambda terminase. The enzyme processively excises and packages individual genomes from a concatemeric DNA precursor, alternating between a stable, site-specifically bound maturation complex and a dynamic packaging motor complex. The terminase complexes are represented as blue balls for simplicity. Biophysical data suggest that the maturation complex is composed of four protomers that encircle the duplex at the cos-site [(TerL₁•TerS₂)₄] (16). A tetrameric complex would thus contain four maturation ATP binding sites and four packaging ATPase sites in the four TerL catalytic subunits, plus eight nucleotide binding sites in the presumed octameric TerS DNA binding ring. Kinetic interrogation of the enzyme has revealed complex allosteric interactions between all of these sites (see (23)). Details are provided in the text.