FIG. 1.

Linear map of domains in gpNu1 and gpA. Relative positions of mutations which affect the endonucleolytic activity of terminase are shown for gpA. ∗, locations of the A-E₅₁₅G, A-N₅₀₉K, and A-R₅₀₄C mutations in the carboxy half of gpA; x, site of an altered amino acid that renders gpA deficient in nicking activity (17); n, residue 497, the site of the A-K₄₉₇D mutation (31); ATP, a proposed ATPase center; bZip, the putative basic leucine zipper region; Pro, the specificity domain for binding to proheads. Residues 484 to 505 comprise the Walker A segment of a putative ATPase domain in gpA. The proposed P-loop region of this domain encompasses residues 485 to 497 (25, 45). In gpNu1, HTH indicates the position of a proposed DNA binding HTH motif. gpNu1 and gpA interact via their carboxy and amino termini, respectively, to form the terminase heteromultimer.