Double-Stranded DNA Bacteriophages

Many bacterial viruses have genomes containing double-stranded DNA. Such viruses were the first bacterial viruses discovered, and have been the most extensively studied. With such a range of double-stranded DNA viruses, a wide variety of replication systems are present. In the present section, we discuss the best studied and most representative of the group, T4 and T7. The simpler, T7, will be discussed first. 

Bacteriophage T7 Bacteriophage T7 and its close relative T3 are relatively small DNA viruses that infect Escherichia coli. (Some strains of Shigella and Pasteurella are also hosts for phage T7.) The virus particle has an icosahedral head and a very small tail. The virus particle is fairly complex, with S different proteins in the head and 3-6 different proteins in the tail. One tail protein, the tail fiber protein, is the means by which the virus particle attaches to the bacterial cell surface. Only female cells of Escherichia coli can be infected with T7 male cells can be infected but the multiplication process is terminated during the latent period. 

Head assembly protein Major head protein Tall protein Tail protein 

RNA polymerase which recognizes the rest of the T7 promoters. We also note that T7 is an example of a virus which strongly affects host transcription and translation processes, by producing proteins 

As seen in the genetic map, the genes after gene 1.1, transcribed by the T7 RNA polymerase, code for proteins that are involved in T7 DNA synthesis, the formation of virus coat proteins, and assembly. Three classes of T7 proteins are formed class I, made 4-8 minutes after infection, which use the cell RNA polymerase class II, made 6-15 minutes after infection, which are made from T7 RNA polymerase and are involved in DNA metabolism class III, made from 6 minutes to lysis, which are transcribed by T7 RNA polymerase and which code for phage assembly and coat protein. This sort of sequential pattern, commonly seen in many large double-stranded DNA phages, results in an efficient channeling of host resources, first toward DNA metabolism and replication, then on to formation of virus particles and release of virus by cell lysis. 

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IV. Large-Scale Irreversible Quaternary Structure Changes in Double-Stranded DNA Bacteriophage... 

Double layered particle (DLP), 82 Double-stranded DNA bacteriophage HK97 and, 209-214 large-scale irreversible quaternary structure changes in, 209-214 Double-stranded DNA viruses, 171-178, 243-253... 

Figure 45. Dark-field micrograph of a small segment of unstained, unshadowed, naked double-stranded DNA from a bacteriophage. A relatively well-preserved image of the two-dimensional projection of the double helix is seen in the left half. The scale bar equals 35 A (116). (Reproduced with kind permission from the Annual Review of Biophysics and Bioengineering, Vol. 8 1979 by Annual Reviews, Inc.)...

The rotational dynamics of ethidium intercalated in double-strand DNAs of intact bacteriophages, namely, X, the deletion mutant XA, T4D (wild type), and T4dC (with normal cytosine instead of glucosylated hydroxymethyl-... 

A sample of double-stranded DNA is denatured. One of the resulting single strands is used as a template to direct the synthesis of a complementary strand of radioactive DNA using a suitable DNA polymerase. The "Klenow fragment" of E. coli, DNA polymerase I, reverse transcriptase from a retrovirus, bacteriophage T7 DNA polymerase, Taq polymerase, and specially engineered enzymes produced from cloned genes have all been used. 

DNA fragments can be introduced into bacterial cells, most or all will lack the ability for self-replication and will quickly be lost. However, two types of DNA molecule are known which can replicate autonomously in bacterial cells bacteriophages (see Topic 14) and plasmids. Plasmids are small circular double-stranded DNA molecules that exist free inside bacterial cells, often carry... 

Double-stranded M13mp2 bacteriophage DNA transferred to E. coli Forward mutations No data + Snow 1991 Snow and Xu 1989 Chromium trichloride... 

Hud, N. V. (1995). Double-stranded DNA organization in bacteriophage heads An alternative toroid-based model. Biophys. J. 69, 1355-1362. 

Lepault, J., Dubochet, J., Baschong, W., and Kellenberger, E. (1987). Organization of double-stranded DNA in bacteriophages A study by cryo-electron microscopy of vitrified samples. EMBO J. 6, 1507-1512. 

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