ATPase center of bacteriophage λ terminase involved in post-cleavage stages of DNA packaging: identification of ATP-interactive amino acids

Terminase is the enzyme that mediates λ DNA packaging into the viral prohead. The large subunit of terminase, gpA (641 amino acid residues), has a high-affinity ATPase activity (Km=5 μM). To directly identify gpA’s ATP-interacting amino acids, holoterminase bearing a His6-tag at the C terminus of gpA was UV-crosslinked with 8-N3-[α-32P]ATP. Tryptic peptides from the photolabeled terminase were purified by affinity chromatography and reverse-phase HPLC. Two labeled peptides of gpA were identified. Amino acid sequencing failed to show the tyrosine residue of the first peptide, E43SAY46QEGR50, or the lysine of the second peptide, V80GYSK84MLL87, indicating that Y46 and K84 were the 8-N3-ATP-modified amino acids. To investigate their roles in λ DNA packaging, Y46 was changed to E, A, and F, and K84 was changed to E and A. Purified His6-tagged terminases with changes at residues 46 and 84 lacked the gpA high-affinity ATPase activity, though the cos cleavage and cohesive end separation activities were near to those of the wild-type enzyme. In virion assembly reactions using virion DNA as a packaging substrate, the mutant terminases showed severe defects. In summary, the results indicate that Y46 and K84 are part of the high-affinity ATPase center of gpA, and show that this ATPase activity is involved in the post-cos cleavage stages of λ DNA packaging.