Poxviridae

Poxviruses have been relegated to the position of relatively unimportant human pathogens after the global eradication of small pox.

They are the largest viruses known and they are very easy to isolate which can be grown in a variety of cell cultures. They produce pocks on the chick chorioallantoic membrane (CAM). These pocks are characteristic, giving their name to all forms of infectious disease "a dose of the pox".

Picture extracted: http://www.stanford.edu/group/virus/pox/2000/index.html

References to text (Poxviridae): http://visp.scripps.edu/pox/PoxWebPages/PoxBackground.aspx http://microbewiki.kenyon.edu/index.php/Poxviridae

Reproductive Cycle of a Poxvirus in a Host Cell

The replication of the poxvirus occurs in the cytoplasm. The virus is sufficiently complex and has acquired all the functions necessary for genome replication.

The penetration of the virus to the cell is complex and involves more than one mechanism. Two stages of uncoating take place. The outer membrane is removed as the particle enters the cell and in the cytoplasm, and the particle is further uncoated when the core passes into the cytoplasm.

Gene expression is carried out by viral enzymes associated with the core. It is divided into two phases: early genes and late genes.

• Early genes: expressed before genome replication
• Late genes: expressed after genome replication

Late promoters are dependent on DNA replication for activity.

The genome replication of the poxvirus involve self-priming, leading to the formation of high m.w. concatemers that are isolated from infected cells which are subsequently cleaved and repaired to make virus genomes. Thymidine kinase and many other virus-encoded enzymes involved in replication offer potential targets for chemotherapeutic agents.

The assembly occurs in the cytoskeleton. Inclusions are formed in the cytoplasm which matures into virus particles. Actin 'comet tails' are formed and they shoot IEV through the cytoplasm to the cell surface, and then into adjacent cells. This movement has been timed and found to be 3µm/min. This may be instrumental in providing an alternate mechanism for cell to cell transmission. The replication of this large, complex virus is rather quick.

Picture extracted: http://visp.scripps.edu/pox/PoxWebPages/PoxBackground.aspx

Genome Structure

The genome is not segmented and it contains a single molecule of linear double-stranded DNA. The complete genome is about 130000-375000 nucleotides long. The genome contains guanine-cytosine and has terminally redundant sequences. The terminally redundant sequences have reiterated inverted terminal sequences which are tandemly repeated. Its genome sequence is repeated at both ends and dsDNA is covalently bonded. dsDNA is cross-linked at both ends.

The viral genome encodes both structural proteins and non-structural proteins. Lipids are present in the envelope. It contributes 4% of the bacterial weight. The composition of viral lipids and host cell membranes are similar.

Virion Structure of a Poxvirus

The virions of poxviruses consist of
· a surface membrane
· a core
· Lateral bodies.
· They may or may not have an envelope.

They produce extracellular and intracellular particles during their life cycle and can occur in two phenotypes. They may be enveloped during their extracellular phase. Extracellular virions initiate the infection. The virus may be sequestered within inclusion bodies that are not occluded and typically contain one nucleocapsid. The virion capsid is enveloped.

Virions are generally ovoid and brick-shaped. They are composed of an external coat containing lipid and tubular or globular protein structures, enclosing one or two lateral bodies and a core. The core can be unilaterally concave, biconcave, or cylindrical with one or two lateral bodies. Lateral bodies are usually lens-shaped and nested between the core membranes, or between the surface membranes.


Thin section of cowpox.














lb - lateral bodies, c - core The bar represents 100 nm.


Thin section of infected chick embryo cell

















pictures extracted: http://www.ncbi.nlm.nih.gov/ICTVdb/WIntkey/Images/em_poxvi.htm