Purification and properties of gene 18 product of bacteriophage T3

Two noncapsid proteins of T3 and T7 phage, the products of gene 18(gp18) and gp19, are required for DNA packaging. By using in vitro complementation for DNA packaging as an assay system, T3 gp18 was purified to near homogeneity from an extract prepared cells infected with a mutant of gene 19(19- extract). The purified gp18 consisted of a single polypeptide having a molecular weight of 10,000, and was eluted as dimers and higher multimers from Sephadex G-75 columns. T7 gp18 was purified by the same procedures as that for T3 gp18 and behaved in the same manner as T3 gp18 throughout all purification steps. Gp18 from either T3 or T7 phage complemented both T3 and T7 18- extract for DNA packaging. These results indicate that, in contrast to gp19 [H. Fujisawa and M. Yamagishi (1981) Prog. Clin. Biol. Res. 64, 239-252], gp18 does not have specificity for T3 or T7 DNA during the in vitro packaging reaction. T3 gp18 was purified from extract containing functional gp19. The gp18 copurified with the gp19 activity. Gp18 and gp19 activities were stable when they were copurified but were unstable when purified separately. These results suggest that gp18 and gp19 function as a complex in the DNA packaging process. The gp18-gp19 preparation had a prohead-stimulated, DNA-dependent ATPase activity.