Dissection of functional domains of the packaging protein of bacteriophage T3 by site-directed mutagenesis

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Intracellular phage T3 DNA is synthesized as a concatemer in which unit-length molecules are joined together in head-to-tail fashion through terminally redundant sequences. During packaging of DNA, mature monomers are cut from the concatemer. The cutting is obligatorily coupled to DNA packaging. The packaging of phage DNA is under the control of a pair of noncapsid proteins, called packaging proteins, gp18 and gp19. gp19 is an ATP-binding protein that plays multiple roles in DNA packaging. gp19 is predicted, from the sequence of its gene, to contain 586 amino acids, and has consensus sequences for an ATP binding site. To dissect structure-function relationships of gp19, mutations were introduced into the ATP binding domain and the mutant proteins were overproduced, purified and characterized. Mutant gp19 with a Gly-to-Asp mutation at amino acid 61 (gp19 G61 D) was defective in DNA packaging due to an altered interaction with ATP. Gp19 G424E, with a change in another putative ATP binding domain, was active in DNA packaging but was defective in DNA cutting. A second mutation in the latter domain, gp19 K430T, and a mutation at 553 (to give gp19 H553L), within a putative Mg 2+ binding domain, had only minor effects on gp19 activities.