Bacteriophage T4

Christopher K Mathews, Oregon State University, Corvallis, USA

Published online: November 2010

DOI: 10.1002/9780470015902.a0000784.pub3

Bacteriophage T4 and its host bacterium, Escherichia coli, can be considered among the earliest model organisms – biological systems that attract large numbers of investigators who, because of technical or conceptual advantages of the system, use the system to investigate processes and mechanisms of general significance in biology. Beginning in the mid-twentieth century, studies focused on bacteriophage T4 revealed essential features of the molecular nature of genes and genomes, mechanism and fidelity of deoxyribonucleic acid (DNA) replication, genetic recombination, DNA repair, control of gene expression, genome organisation, assembly of complex macromolecular structures and pre-emption of cell metabolism by virus infection. Although much of the molecular biology research community has moved on to eukaryotic model organisms, such as yeast, nematode worms, fruitflies, the plant Arabidopsis, zebrafish and mice, bacteriophage T4 continues its service, both as an experimental system and as a source of research reagents and technologies.

Key Concepts:

  • Bacteriophage T4 presents numerous technical advantages as a model for studying virus reproduction or gene organisation and expression.
  • Bacteriophage T4 is representative of a large group of closely related phages found in varied environments worldwide.
  • Bacteriophage T4 is a virulent phage, which always lyses and kills its host bacterium.
  • The T4 virion has a complex multiprotein structure with a contractile tail that serves both for adsorption to host cells and for intracellular delivery of the viral genome.
  • Bacteriophage T4 contains a large, linear double-stranded DNA genome, with chemical modifications of its cytosine residues.
  • The T4 genome is circularly permuted and terminally redundant with respect to base sequence; these features protect against information loss during replication of a linear DNA.
  • The T4 genome encodes numerous enzymes, used to support replication of the viral genome and to synthesise deoxyribonucleotides to support the enormous rate of DNA accumulation in infected cells.
  • T4 uses host cell RNA polymerase for transcription of its own genes, but it modifies the bacterial enzyme, both to prevent its transcription of bacterial genes after infection and to contribute towards a timed viral gene transcription program.
  • T4 morphogenesis involves separate subassembly pathways for viral heads, tails, tail baseplates and fibers, with the substructures assembling spontaneously.

Keywords: bacteriophage; genetics; molecular biology; viruses; enzymes; model organisms

Figure 1. A model of the T4 virion, with its tail contracted, based on image reconstruction from electron microscopic images. The icosahedral head (in grey) contains four exterior proteins and within its interior, DNA and six different proteins. The collar and portal structure (in orange and blue) contains six proteins. The whiskers (not shown), which are attached to the collar, contain one protein. The contractile tail sheath (in green) contains multiple copies of one ATP-binding protein. The tail tube (in pink) contains one protein. The complex tail baseplate (in red, yellow and blue) contains a central hub, with eight proteins, and six surrounding wedges, each with six proteins. The six short tail fibres (in purple) contain four proteins each. The six long-tail fibres (in light blue) contain four proteins each. Reprinted from Leiman et al. (2004), with permission from Elsevier.
Figure 2. Generation of a population of linear DNA molecules that are circularly permuted and terminally redundant with respect to base sequence, by cutting of a concatemeric replication intermediate. In this scheme the genome is depicted as the alphabet. If the cutting mechanism recognises slightly more than one genome length, each resultant DNA molecule has two letters (as schematised) repeated at the beginning and end of the molecule.
Figure 3. (a) A one-step growth curve for bacteriophage T4 infecting E. coli B at 37°C. Blue circles, infective centres; red circles, intracellular phages; see text for details. PFU, plaque-forming unit. (b) A chronology of major events in T4 infection. The precise times of expression of genes within the major temporal classes vary somewhat with individual genes, as shown. Under some conditions, lysis may occur later than 30 min after infection; under those conditions, replication, transcription, translation and viral assembly continue until lysis. Reprinted from Mathews CK (1994) In: Karam JD (ed.) Molecular Biology of Bacteriophage T4, p. 2. Washington DC: American Society for Microbiology, with permission from Elsevier.
Figure 4. Overview of the T4 developmental programme. Created by FA Eiserling and reprinted from Mathews CK (1994) In: Karam JD (ed.) Molecular Biology of Bacteriophage T4, p. 5. Washington DC: American Society for Microbiology, with permission from Elsevier.
Figure 5. A model of the T4 phage replisome, showing the actions of DNA polymerase (gp43), a dimeric enzyme extending both leading and lagging DNA strands; trimeric processivity clamp (gp45), which binds polymerase to its template strand; clamp loader complex (gp44/62), which attaches gp45 to DNA; hexameric helicase (gp41), which unwinds parental DNA strands; helicase loader (gp59), which recruits gp41 to its binding site on DNA; hexameric primase (gp61), which synthesises pentanucleotide RNA primers and single-strand DNA-binding protein (gp32), which binds template DNA in extended single-strand configuration, so that it can base pair with incoming nucleotides. Gp41 unwinds parental DNA, followed by action of gp61 to synthesise pentameric RNA primers, which are then handed off to DNA polymerase. Gp59 loads gp41 helicase onto DNA at the replication fork. The single-strand DNA generated during DNA unwinding is coated by monomeric gp32. The gp45 clamp is loaded onto DNA by the gp44/62 complex. The leading and lagging strand DNA polymerases are tethered to form a dimer at the replication fork. Reprinted from SK Perumal et al. (2010), with permission from Elsevier, Inc.
Figure 6. The T4 DNA packaging motor. (a) The position of the portal protein gp20 (in red) and the gp17 packaging motor protein gp17 (in orange and green) with respect to the prohead. (b) The domain structure of gp17, showing the ATPase ‘engine’ domain in orange and the nuclease-containing ‘wheel’ protein, the rotation of which with respect to DNA drives it into the prohead. Reprinted from Sun et al. (2008), with permission from Elsevier, Inc.
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    book Karam JD (ed.) (1994) Molecular Biology of Bacteriophage T4. Washington DC: American Society for Microbiology.
    book Mathews CK, Kutter EM, Mosig G and Berget P (eds) (1983) Bacteriophage T4. Washington DC: American Society for Microbiology.

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DNA Viruses | Viruses Infecting Bacteria
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Article Title: Bacteriophage T4
 
How to Cite close
Mathews, Christopher K(Nov 2010) Bacteriophage T4. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000784.pub3]