Bacteriophages: Structure


Bacterial viruses, or bacteriophages, are ubiquitous organisms spanning very different ecological niches. Although genome comparison fails to show extensive relationship among bacteriophages, recent structural studies reveal a high degree of similarities. Most bacteriophages present an icosahedral proteinaceous head, which contains the nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Exceptions to this rule are few cases where a lipid envelope forms part of the head, and those other cases where the head presents a filamentous geometry. The way bacteriophages infect the host cell is the basis of a main difference among them: one group (Caudovirales) has a specialised structure (the tail) that is responsible for the recognition of the host cell and the viral genome delivery, whereas those bacteriophages without tail present a variety of infecting strategies. In this study we will deal with the main common characteristics of bacteriophage heads and tails supporting their common evolutionary origin.

Key Concepts:

  • Bacteriophages share extended structural and functional similarities.

  • Bacteriophages are excellent examples of optimisation of genetic information to carry out complex functions.

  • Most bacteriophages enclose their nucleic acid in a protein container (built by multiple copies of one (or a few) proteins), and in a few cases they may include lipid envelopes.

  • The most conserved geometry in bacteriophage heads is icosahedral.

  • Double stranded DNA icosahedral bacteriophages follow a common assembly pathway.

  • The Caudovirales group (bacteriophages with icosahedral heads, dsDNA and a tail) are the most abundant virus type.

  • The packaging of dsDNA inside Caudovirales requires energy conversion (ATP hydrolysis) into mechanical translocation, and it is carried out by a dedicated viral machinery.

  • The maturation of icosahedral proheads into viral heads involves drastic reorganisations of the capsid components leading to more stable particles.

  • The tail of Caudovirales is a sophisticated machinery involved in cell recognition and nucleic acids delivery.

  • There are three main architectural designs of bacteriophage tails, each one adapting to different host interaction strategies.

Keywords: bacteriophage; structure; phage assembly; bacterial virus; maturation

Figure 1.

Structure of bacteriophage proheads. (a) Three‐dimensional reconstruction of bacteriophage T7 proheads, this research was originally published in Ionel et al., the American Society for Biochemistry and Molecular Biology Inc. (b) Structure of P22 prohead at 0.4 nm resolution (Chen et al., ). (c) P22 prohead showing the singular 5‐fold vertex where the portal attaches (Reproduced with permission from Chen et al., copyright, PNAS). (d) Central section of the P22 prohead showing the internal scaffold (red) and the portal‐core complex (grey) (Reproduced with permission from Chen et al., copyright, PNAS). The bar represents 20 nm. (e), ribbon representation from the structures of the major shell protein of bacteriophages HK97 (PDB 1ohg) (upper), T7 (PDB 3izg) (middle) and T4 (PDB 1yue) (lower).

Figure 2.

Structure of connector complexes. (a) P22 connector. Reproduced with permission from Tang et al. copyright, Elsevier. (b) T7 connector. Reproduced with permission from Agirrezabala et al. copyright, Elsevier. (c) Ribbon representation of the connector proteins of phage ϕ29, SPP1 and C‐terminal truncated P22 (from left to right). Reproduced with permission from Cuervo and Carrascosa copyright, Elsevier. The dotted area marks the common α helical structural signature of the connector protein.

Figure 3.

Structure of mature bacteriophage. (a) Structure of P22 mature virion. Reproduced with permission from Tang et al. copyright, Elsevier. Head is depicted in blue, and the tail components in yellow, pink and cyan. (b) Central section of the P22 virion showing the outline of the shell (blue), DNA layers (green), portal complex (red) and tail components (yellow, pink, blue and orange). Reproduced with permission from Tang et al. copyright, Elsevier. (c) Detail of the interaction between the connector (grey) and the capsid shell (blue). The DNA is depicted in green along the portal channel (Reproduced with permission from Tang et al., copyright, Elsevier). (d) High resolution (0.45 nm) model of the ɛ‐15 bacteriophage shell. The seven subunits of the asymmetric unit are labelled with different colours. Reproduced with permission from Jiang et al., copyright, Nature Publishing Group.

Figure 4.

Structural modifications involved in the maturation of the bacteriophage shell. (a) and (b) Central sections of, respectively, the prohead and mature head shells of phage T7. Reproduced with permission from Ionel et al., copyright, The American Society for Biochemistry and Molecular Biology Inc. (c) Relative movements of the structural domains of the major shell protein of phage HK97 which take place during head maturation. Prohead subunit is yellow and head subunit is green. The invariant P‐domain in both subunits is depicted in blue. Reproduced with permission from Gertsman et al., copyright, Naure Publishing Group. (d) and (e) Asymmetric units of prohead and mature head of T7, respectively. The translucent electron density model is fitted in each case with the ribbon representation of the major shell protein. Reproduced with permission from Ionel et al., copyright, The American Society for Biochemistry and Molecular Biology Inc.

Figure 5.

(a) Negative staining electron microscopy images of bacteriophages belonging to the three Caudovirales families from left to right SPP1 (Syphoviridae). Reproduced with permission from Plisson et al., copyright, Nature Publishing Group. T4 (Myoviridae) and ϕ29 (Podoviridae). The bar represents 50 nm. (b) Three‐dimensional electron microscopy reconstructions obtained from different bacteriophage tails. Upper left panel, fragment of P2 tail (Syphoviridae) showing the tail tube (blue) and the baseplate (yellow). Reproduced with permission from Veesler and Cambillau , copyright, The American Society of Microbiology. Bottom left panel, final part of the SPP1 tail (Syphoviridae), the tail cap (green) and the tail tip (pink) are shown. Reproduced with permission from Plisson et al., copyright, Nature Publishing Group. Central panel, T4 tail (Myoviridae) in a contracted conformation showing the sheath (gp18 in green), the tail tube (gp19 in pink), the baseplate (gp7–12, in red, dark blue, yellow, light green, light blue and purple, respectively), the fibres (gp14 in white) and the neck (gp13‐15 in light purple and orange, respectively). Reproduced with permission from Leiman et al., copyright, Elsevier. Upper right panel, ϕ29 tail (Podoviridae), showing the appendages in purple, the collar in green and the knob in blue. Reproduced with permission from Tang et al., copyright, Elsevier. Bottom panel, P22 tail (Podoviridae) showing the tail adaptor protein (pink), the tail knob (green), the spikes (blue) and the needle (yellow). Reproduced with permission from Lander et al., copyright, Elsevier.



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Vinga I, Sao‐José C, Tavares P and Santos M (2006) Bacteriophage entry in the host cell. In: Wegrzn G (ed.) Modern Bacteriophage Biology and Biotechnology, pp. 165–205. Kerala: Research SignPost.

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Cuervo, Ana, and Carrascosa, José L(Jun 2012) Bacteriophages: Structure. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0024053]