Lytic cycle

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Schematic sequence of a lytic cycle:
1) Adsorption
2) Injection
3) Bacterial DNA synthesis is stopped and phage components are built up
4) Lysis

In viruses, the lytic cycle describes the developmental phase in which the host cell is lysed after new virions have been formed. This creates a cytopathic effect . Viruses that maintain their host cells in the lytic cycle represent a borderline case because they attach themselves directly to the cell membrane and leave the host cell by constricting a vesicle ( budding of non-lytic viruses). The lysogenic cycle , on the other hand, maintains the host cell and is usually associated with a reduced synthesis of viral genes and is sometimes referred to as the resting phase or latent phase. Viruses with a violent but transient course of infection usually do not have a lysogenic cycle ( hit and run viruses ).

General cycle course

  • Host cell - contact with virus ( adsorption )
  • “Injection” of the phage genome or penetration of the cell or endosome membrane
  • Replication of the viral genome ( DNA or RNA ) or, in the case of retroviruses , only the reverse transcription takes place
  • Integration of the virus genome into the host genome or, in the case of retroviruses, first the reverse transcription into DNA, then the integration. RNA viruses (except retroviruses) and some DNA viruses do not integrate. In the case of bacteriophages, integration is sometimes also assigned to the lysogenic cycle.
  • Gene expression to build up the replication enzymes and the packaging material, possibly a modulation of the host restriction and (in animals) also the immune system
  • Packaging of the genome for the generation of daughter viruses, with some viruses replication takes place at the same time as packaging (e.g. hepatitis B virus )
  • Lysis of the host cell or budding of the virions
  • Infection of other cells

Typical lytic viruses are e.g. B. adenoviruses , the hepatitis A virus , the hepatitis E virus , influenza viruses , the measles virus , smallpox virus , the poliovirus , rhinoviruses , rotaviruses , the rubella virus , the SARS coronavirus , the rabies virus and the tobacco mosaic virus .

Course of the cycle in bacteriophages

The cycle of lytic reproduction of a bacteriophage is associated with the death of the host cell. During the lysis, the newly produced viruses are released and can attack new cells again; they are then referred to as virulent phages. The lysed bacterium releases between 100 and 200 phages ( burst size ). A phage species that only rarely lyses and is mainly in the lysogenic cycle is called temperate .

The lytic cycle of E. coli with the T4 phage lasts 20 to 30 minutes at 37 ° C. The process for an even-numbered phage of the T series is described below; other phages may show different processes in some details. The lytic infection cycle runs exactly according to the scheme described under 3.): If a phage encounters a suitable host cell and comes into contact with it, it adsorbs specific cell wall components of the host cell with tail fibers and spikes. These receptors are, among other things, antigen structures , lipoproteins or lipopolysaccharides on the host cell surface for which the corresponding phage has an affinity. This affinity also explains the strict host specificity that phages are subject to - only the phage whose endplate, tail fiber and spike proteins match the receptor on the host cell surface can "dock" to the cell. If the receptor is naturally missing (because it is the wrong bacterium) or if it has been lost through mutation, the cell is insensitive, i.e. resistant to infection by the phage in question. This can lead to resistant clones .

If the phage is adsorbed on the cell wall (the process is irreversible and thus similar to an antigen-antibody reaction), the tail sheath contracts and the tail tube penetrates the cell wall at the same time. This process prepares the injection of the phage DNA and is supported by the simultaneous release of lytic phage enzymes , which make the cell wall permeable. When the phage genome is injected, the empty protein shell consisting of capsid and injection device remains on the surface of the host cell. This remaining virus particle without DNA is called a ghost .

Immediately after the injection of the phage DNA, no phages or no phage particles can be detected in the bacterial cell; this period is called an eclipse or latency period . With Escherichia coli , this latency period is around 10 minutes. During this time nothing seems to happen in the cell. In fact, during this phase the entire metabolism of the host cell is switched to the production of phage components. Cell division and the synthesis of bacterial DNA are stopped immediately after the injection of the viral DNA, in that the viral DNA is transcribed into an mRNA with the help of the cell's own enzymes and proteins are synthesized that interrupt the bacterial DNA synthesis, the already existing bacterial Break up DNA into fragments and use them to synthesize new phage DNA. In the further course of the infection, so-called “late” genes are also transcribed, which cause the synthesis of structural proteins (such as head, tail pipe, tail fibers, etc.). These components are first assembled individually in the host cell and then assembled into new phages. The appearance of the first assembled phage ends the eclipse. From now on, phages are assembled until the internal pressure of the host cell exceeds a certain level and the cell wall is torn open by a lysozyme- like enzyme, which is synthesized parallel to the assembly of the phage particles. This releases the newly formed phages. This process is known as lysis.

The replication of the DNA of some phages is based on the rolling circle model. A strand of DNA is cut open and unrolled by adding new nucleotides to this strand. The ring, which is still closed, serves as a supply line during unwinding. The result is a concatemer, i.e. a long strand of DNA which breaks down when the DNA is packaged into the phage genome.

See also

literature

  • Michael T. Madigan (Author), John M. Martinko (Author), Thomas Lazar (Translator), Freya Thomm-Reitz (Translator): Brock Microbiology. 11th edition. Pearson studies, Munich / Boston et al. 2008, ISBN 978-3-8273-7358-8 .
  • David M. Knipe, Peter M. Howley, Diane E. Griffin, (Eds.): Fields Virology. 5th edition, Lippincott Williams & Wilkins, Philadelphia 2007, ISBN 978-0-7817-6060-7 .

Individual evidence

  1. ↑ bacteriophages. Retrieved January 16, 2020 .
  2. ↑ Rolling Ring Replication. Retrieved January 15, 2020 .