Bacteriophage T7 morphogenesis: Phage-related particles in cells infected with wild-type and mutant T7 phage

Abstract

Extracts from cells infected with wild-type T7 phage were found to contain three classes of particles: proheads, empty heads, and mature phage. These three particles differed with respect to their sedimentation properties, protein composition, and appearance in the electron microscope. All three structures contained the proteins specified by genes 8, 10, 13, 14, 15, and 16. Proheads and empty heads also contained the proteins coded for by genes 9 and 19. However, proheads contained more of the gene 9 product and less of the gene 19 product than empty heads. Phage did not contain the products of genes 9 or 19 but did contain three tail proteins, those specified by genes 11, 12, and 17. The product of gene 18 was found only in empty heads. In the electron microscope, proheads appeared to exclude the negative stain to a considerable degree, were round in outline, had thick shells, and contained eccentrically placed cores. Empty heads were easily penetrated by stain, had thinner shells than proheads, and often appeared to be collapsed or broken. Phage had polyhedral heads and small conical tails. Lysates of Su° cells infected with phage bearing amber mutations were examined for the presence of head-related structures. In the absence of the products of gene 9 or 10, no head-like structures were produced. After infection with a mutant in gene 8, some proheads but no empty heads or full heads were synthesized. Mutants in genes 14, 15, and 16 produced proheads, empty heads, and phage, although in reduced amounts relative to wild-type. The phage produced were noninfectious. The proheads produced after infection with mutants in genes 8, 14, 15, and 16 had aberrant protein compositions and appeared to lack cores when examined in the electron microscope. In the absence of the products of genes 18 or 19, or in the absence of T7 DNA, proheads accumulated but no other head structures were formed. After infection with mutants in genes 11, 12, and 17, head assembly and DNA packaging proceeded normally but tail assembly was defective. In the absence of the proteins specified by genes 7 and 13, phage assembly proceeded efficiently but the phage-like particles which were formed were noninfectious. On the basis of these data, we have proposed a model for phage T7 assembly in which the prohead, in the presence of the products of genes 18 and 19, packages DNA and loses P9 to give a full head to which are added the tail proteins. According to this model, the empty head is a breakdown product of a prohead which has initiated but not yet completed DNA packaging.