Genetic studies on capsid-length determination in bacteriophage T4. I. Isolation and partial characterization of second-site revertants of a gene 23 mutation affecting capsid length

On the structure of the scaffolding core of bacteriophage T4

The scaffolding core in bacteriophages is a temporary structure that plays a major role in determining the shape of the protein shell that encapsulates the viral DNA. In the currently accepted structure for the scaffolding core in bacteriophage T4, there is a symmetry mismatch between the protein shell, which has fivefold symmetry, and the scaffolding core, which is believed to consist of six helical chains. The analysis of T4 giant prohead data that was used to determine this structure made an implicit assumption about the manner in which giant proheads flatten during preparation for electron microscopy. Namely, it was assumed that techniques for analysis of Fourier transforms of flattened single-layer cylinders could be applied independently to the shell and the core. This analysis makes the implicit assumption that connections between the core and the shell do not affect the flattening process, and thus are stretched or broken during the flattening process. Reexamination of the experimental data shows that this assumption is likely to be incorrect. A reanalysis shows that the data could be consistent with six, eight, or 10 helical chains, and is a better match for eight or 10 helical chains. Ten helical chains would match the fivefold symmetry of the shell. The 10-helix core model is particularly attractive because it suggests a Vernier mechanism, which is able to explain the process of length determination in giant head mutants of T4. It is possible that the same assumption has been made for structural analysis of other biological systems. If this is the case, any results obtained should also be reexamined.

On the Structure of the Scaffolding Core of Bacteriophage T4 and Its Role in Head Length Determination

The scaffolding core in bacteriophages is a temporary structure that plays a major role in determining the shape of the protein shell that encapsulates the viral DNA. In the currently accepted structure for the scaffolding core in bacteriophage T4, there is a symmetry mismatch between the protein shell, which has fivefold symmetry, and the scaffolding core, which is believed to consist of six helical chains. Alternate structures for the scaffolding core in T4 are investigated. Starting with the hypothesis that the shell and a 10-helix core would have matching symmetry, a Vernier mechanism is proposed that explains the previously unexplained behavior of the length determination process in giant head mutants of T4. Other possible Vernier mechanisms for core structures containing six and eight helices are also explored. Copyright 1998 Academic Press.

Mutations that eliminate the requirement for the vertex protein in bacteriophage T4 capsid assembly

The capsid of bacteriophage T4 is composed of two essential structural proteins, gp23, the major constituent of the capsid, and gp24, a less prevalent protein that is located in the pentameric vertices of the capsid. gp24 is required both to stabilize the capsid and to allow it to be further matured. This requirement can be eliminated by bypass-24 (byp24) mutations within g23. We have isolated, cloned and sequenced several new byp24 mutations. These mutations are cold-sensitive in the absence of gp24, and are located in regions of g23 not known to contain any other mutations affecting capsid assembly. The cold-sensitivity of the byp24 mutations can be reduced by further mutations within g23 (trb mutations). Cloning and sequencing of these trb mutations has revealed that they lie in regions of g23 that contain clusters of mutations that cause the production of high levels of petite and giant phage (ptg mutations). Despite the proximity of the trb mutations to the ptg mutations, none of the ptg mutations has a Trb phenotype. The mutation ptE920g, which is also located near one of the ptg clusters, and which produces only petite and wild-type phage, has been shown to confer a Trb but not a Byp24 phenotype. The relevance of these observations to our understanding of capsid assembly is discussed.

Genetic control of capsid length in bacteriophage T4. VII. A model of length regulation based on DNA size

Four models for head length regulation in bacteriophage T4 are described and discussed. Several length mutants in the major capsid protein gene (23) were studied by sucrose gradient analysis, rotating gel analysis of DNA length, and by mixed infection gene dosage experiments with T4 amber mutants in gene 24. The results show that head length variation is quantized and highly specific, in that certain amino acid changes in gp23 results in reproducible and well-defined head length phenotypes. These data are presented as being most consistent with a vernier-type of head length control mechanism.

Quantized viral DNA packaging revealed by rotating gel electrophoresis

Two classes of missense mutations in the bacteriophage T4 gene coding for the major head protein produce phage with different length heads. The pt (petite) mutations produce phage with normal, intermediate, and isometric heads, whereas ptg (petite and giant) mutations also produce greatly elongated (giant) heads. DNA from petite, normal, and giant particles was clearly resolved by discontinuous rotating gel electrophoresis, and several new species of headful length DNA were found. These results confirm the idea that the major stop points for head length regulation are at Q = 13, 17, and 21, and also show that minor stop points exist at Q = 16, 18 and 20. The existence of these well-defined classes of DNA that correlate with capsid structure suggest that a structural relationship between the scaffold protein and the capsid protein determines head length and thus DNA length.

Genetic control of capsid length in bacteriophage T4: clustering of ptg mutations in gene 23

Fifty-two new bacteriophage T4 ptg mutations have been isolated by selecting for the giant-capsid phenotype they display. Genetic mapping placed all of them at eight sites, all located in gene 23. These sites were clustered in three locations, one near amber B17 (gene 23 nucleotide [NT] 268), another centrally placed between amE506 (NT 706) and amE1270 (NT 925), and the third between amC208 (NT 1297) and amE1236 (NT 1489). The lack of a selective system for identifying recombinant genotypes when dealing with the very close linkages found within these clusters opens the possibility that more than eight sites are represented in this set of mutations. Since one site was represented by only one mutation, it seems likely that further searching might uncover additional sites. It is suggested that the clustering of mutations observed here identifies regions of the gene 23 product that play a role in regulating the capsid length of T4.

Genetic control of capsid length in bacteriophage T4: DNA sequence analysis of petite and petite/giant mutants

The T4 gene 23 product (gp23) encodes the major structural protein of the mature capsid. Mutations in this gene have been described which disrupt the normal length-determining mechanism (A.H. Doermann, F.A. Eiserling, and L. Boehner, J. Virol. 12:374-385, 1973). Mutants which produce high levels of petite and giant phage (ptg) are restricted to three tight clusters in gene 23 (A.H. Doermann, A. Pao, and P. Jackson, J. Virol. 61:2823-2827, 1987). Twenty-six of these ptg mutations were cloned, and their DNA sequence alterations were determined. Each member of this set of ptg mutants arose from a single mutation, and the set defined 10 different sites at which ptg mutations can occur in gene 23. Two petite (pt) mutations in gene 23 (pt21-34 and ptE920g), which produce high frequencies of petite particles but no giants, were also sequenced. Both pt21-34 and ptE920g were shown to include multiple mutations. The phenotypes attributed to both pt and ptg mutations are discussed relative to the mechanism of capsid morphogenesis. A site-directed mutation (SD-1E) was created at the ptgNg191 site, and its phenotypic consequences were examined. Plaque morphology revertants arising from a gene 23 mutant derivative of pt21-34 and from SD-1E were isolated. A preliminary mapping of the mutation(s) responsible for their revertant phenotypes suggested that both intra- and extragenic suppressors of the petite phenotype can be isolated by this method.

Genetic control of capsid length in bacteriophage T4: phenotypes displayed by ptg mutants

The phenotypic characteristics of 26 ptg mutations in T4 gene 23 are described. All were located in three tight clusters in that gene and, by definition of ptg mutations, all produced giant phage. Intermediate petite phage, which invariably made up a substantial fraction of the progeny of these mutants, appeared to be a unique product of gene 23 mutations. Isometric petite phage were produced in significant numbers by strains with mutations at only 4 of the 10 sites identified with the PTG phenotype. The data presented indicate that there was little if any variation in the lengths of the normal, the intermediate petite, and the isometric petite classes. The frequencies of those capsid types were fairly specific for the individual mutations. The giant capsids that resulted from ptg mutations also had characteristic length distributions, of which three types were distinguished. These highly specific effects of gene 23 ptg mutations on capsid length regulation of T4 imply that the product of gene 23, gp23, plays a significant role in controlling the length of its capsid. The restrictions these observations place on a model for T4 capsid length regulation are discussed briefly.

Genetic studies on capsid-length determination in bacteriophage T4. II. Genetic evidence that specific protein-protein interactions are involved

A bacteriophage T4 mutation (ptg19-80c) located in gene 23, which encodes the major structural protein of the T4 capsid, results in the production of capsids of abnormal length. Mutations outside gene 23 which partially suppress ptg19-80c have been described in the accompanying paper (D. H. Doherty, J. Virol. 43:641-654, 1982). Characterization of these suppressors was extended. A complementation test suggested that the suppressors were in genes 22 and 24. These genes coded for the major component of the morphogenetic core of the capsid precursor and the vertex protein of the capsid, respectively. The suppressor mutations were found to have no obvious phenotype in the absence of ptg19-80c. Suppression was shown to be allele specific: other ptg mutations at different sites in gene 23 were not suppressed by the suppressors of ptg19-80c. These results indicated that specific interactions among the three proteins gp22, gp23, and gp24 may play a role in the regulation of T4 capsid-length determination. Current models for capsid-length determination are considered in the light of these results.