Replication cycles in bacteriophages

Viruses that infect bacterial cells are called bacteriophages (phages for short), which means, literally, 'bacteria eaters'. Perhaps the best understood of all viral replication cycles are those of a class of bacteriophages which infect E. coli, known as the T-even phages. These are large, complex viruses, with a characteristic head and tail structure (Figure 10.8). The double-stranded, linear DNA genome contains over 100 genes, and is contained within the icosahedral head. The growth cycle is said to be lytic, because it culminates in the lysis (=bursting) of the host cell. Figure 10.9 shows the lytic cycle of phage T4, and the main stages are described below.

Even Phage
Figure 10.8 A T-even bacteriophage. Note the characteristic 'head plus tail' structure. The tail fibres and base plate are involved in the attachment of the phage to its host cell's surface. Reproduced by permission of Professor Michael J Pelczar, University of Maryland
Phage Particles Pic
Figure 10.9 The lytic cycle of phage T4. The cycle comprises the five main stages described in the text; from injection of phage of DNA to cell lysis takes 22 minutes. The number of phage particles released per cell is called the burst size, and for T4 it ranges from 50 to 200

1 Adsorption (attachment): T4 attaches by means of specific tail fibre proteins to complementary receptors on the host cell's surface. The nature of these receptors is one of the main factors in determining a virus's host specificity.

2 Penetration: The enzyme lysozyme, present in the tail of the phage, weakens the cell wall at the point of attachment, and a contraction of the tail sheath of the phage causes the core to be pushed down into the cell, releasing the viral DNA into the interior of the bacterium. The capsid remains entirely outside the cell, as elegantly demonstrated in the famous experiment by Hershey and Chase (see Chapter 11).

You might reasonably ask yourself why cells would evolve receptor molecules for viruses, when the outcome is clearly not in their interests. The answer is, of course, that they haven't; the receptors have other biological properties, but the viruses have 'taken them over'.

Replication: Phage genes cause host protein and nucleic acid synthesis to be switched off, so that all of the host's metabolic machinery becomes dedicated to the synthesis of phage DNA and proteins. Host nucleic acids are degraded by phage-encoded enzymes, thereby providing a supply of nucleotide building blocks. Host enzymes are employed to replicate phage DNA, which is then transcribed into mRNA and translated into protein.

Assembly: Once synthesised in sufficient quantities, capsid and DNA components assemble spontaneously into viral particles. The head and tail regions are synthesised separately, then the head is filled with the DNA genome, and joined onto the tail.

Release: Phage-encoded lysozyme weakens the cell wall, and leads to lysis of the cell and release of viral particles;these are able to infect new host cells, and in so doing recommence the cycle. During the early phase of infection, the host cell contains components of phage, but no complete particles. This period is known as the eclipse period. The time which elapses between the attachment of a phage particle to the cell surface and the release of newly-synthesised phages is the latent period (sometimes known as the burst time); for T4 under optimal conditions, this is around 22 minutes. This can be seen in a one-step growth curve, as shown in Figure 10.10.

'Early' proteins are viral enzymes including DNA polymerase, used to syn-thesise more phage DNA, and others that disrupt normal host processes. Later, production switches to 'late', structural proteins, required for the construction of capsids; these are produced in much greater quantities.

Eclipse Period Viral Growth Curve

Figure 10.10 The one-step growth curve. During the eclipse period the host cell does not contain complete phage particles. Following synthesis of new particles, they are released, signalling the end of the latent period. The left-hand curve represents the number of phage particles, while the number of free (extracellular) particles is shown by the right-hand curve

Figure 10.10 The one-step growth curve. During the eclipse period the host cell does not contain complete phage particles. Following synthesis of new particles, they are released, signalling the end of the latent period. The left-hand curve represents the number of phage particles, while the number of free (extracellular) particles is shown by the right-hand curve

Phage DNA injected into host cell

Phage DNA replicates

Phage DNA integrates into host chromosome as prophage

Phage DNA replicates

Phage DNA integrates into host chromosome as prophage

Phage Growth Curve

Prophage replicated along with bacterial DNA

Figure 10.11 Replication cycle of a temperate phage. In the lysogenic pathway, the phage DNA is integrated as a prophage into the host genome, and replicated along with it. Upon induction by an appropriate stimulus, the phage DNA is removed and enters a lytic cycle

Prophage replicated along with bacterial DNA

Figure 10.11 Replication cycle of a temperate phage. In the lysogenic pathway, the phage DNA is integrated as a prophage into the host genome, and replicated along with it. Upon induction by an appropriate stimulus, the phage DNA is removed and enters a lytic cycle

Lysogenic replication cycle

Phages such as T4, which cause the lysis of their cells, are termed virulent phages. Temperate phages, in addition to following a lytic cycle as outlined above, are able to undergo an alternative form of growth cycle. Here, the phage DNA actually becomes incorporated into the host's genome as a prophage (Figure 10.11). In this condition of lysogeny, the host cell suffers no harm. This is because the action of repressor proteins, encoded by the phage, prevents most of the other phage genes being transcribed. These genes are, however, replicated along with the bacterial chromosome, so all the bacterial offspring contain the incorporated prophage. The lysogenic state is ended when the survival of the host cell is threatened, usually by an environmental factor such as UV light or a chemical mutagen. Inactivation of the repressor protein allows the phage DNA to be excised, and adopt a circular form in the cytoplasm. In this form, it initiates a lytic cycle, resulting in destruction of the host cell. An example of a temperate phage is bacteriophage X (Lambda), which infects certain strains of E. coli. Bacterial strains that can incorporate phage DNA in this way are termed lysogens.

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Responses

  • mirrin
    What is the period called when phage particle is released ?
    4 years ago
  • lotho
    What is bacteriophage interchanged replication cycle?
    3 years ago
  • anna
    What is the replication time of a phage?
    11 months ago

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