In this article we will discuss about the structure and replication of pox viruses.
Structure of Pox Viruses:
Oval or brick-shaped 200-400 nm long particles can be visualized by the best light microscopes. The external surface is ridged in parallel rows, sometimes arranged helically (Fig. 17.17). Viral particles (virions) are generally enveloped (external enveloped virion-EEV).
The intracellular mature virion (IMV) form of the virus contains different envelope and is also infectious. On the basis of their species they vary in their shape but generally appear brick like or as an oval form similar to a rounded brick.
The virion size is around 200 nm in diameter and 300 nm in length and carries its genome in a single, linear, double-stranded segment of DNA. The external surface of the virion is ridged in parallel rows, sometimes arranged helically. The particles are extremely complex and contain more than 100 different proteins. Antigenically, poxviruses are very complex inducing both specific and cross-reacting antibodies.
Replication of Pox Viruses:
Viral genome is a linear 130-300 kb long (ds) DNA molecule. Ends of genome consist of a terminal hairpin loop (no free ends) with several tandem (i.e. direct) repeat sequences. This arrangement is found at the ends of chromosomes from a number of different organisms.
The ends of the genome form repeats called ‘inverted terminal repeats’ (ITRs) (Fig. 17.18). Several poxvirus genomes have been sequenced. Most of the essential genes are located in the central part of the genome, whereas non-essential (in tissue culture) genes are located at the ends. There are about 250 genes in the genome of poxviruses.
Replication of the poxvirus is accomplished in several stages. Initially, the virus binds to a receptor present on the surface of host cell. So far the receptors for the poxvirus are not known.
Thereafter, the virus enters inside the cell where its uncoating is done in two steps:
Firstly, the outer membrane is removed as the particle enters the cell.
Secondly, the virus particle is further uncoated to release the core into the cytoplasm (without the outer membrane) (Fig. 17.19). The genes of pox virus are expressed in two phases; the early genes are expressed first. These genes encode the nonstructural protein including proteins necessary for replication of the viral genome and are expressed before replication of genome.
The late genes are expressed after genome replication. The structural proteins of the genome make the virus particle. Assembly of the virus particle is a complex process which is poorly understood. It occurs in cytoskeleton of the cell. This virus is large and complex; its replication is relatively quick which is completed in about 12 hours.
Its replication is unusual compared to other viruses with double stranded DNA genome because it encodes its own machinery for genome replication; therefore, its replication occurs in the cytoplasm. However, most of the viruses consisting of a double stranded DNA genome replicate inside the nucleus and use the genome replication machinery of the host cells.
The prototype of poxvirus family is vaccinia virus, which has been used as a successful vaccine to eradicate smallpox virus. Vaccinia virus is also used as an effective tool for foreign protein expression to elicite strong host immune response. Vaccinia virus enters cells mainly by cell fusion, although currently the receptor is not known.
Virus contains three classes of genes: early, intermediate and late genes which are transcribed by viral RNA polymerase and associated transcription factors. Vaccinia virus replicates its genome in cytoplasm of the infected cells and after late gene expression virion morphogenesis produces IMV that Contains envelope, although the origin of the envelope membrane is still unknown.
IMV is transported to Golgi to be wrapped by additional two membranes to become intracellular enveloped virus (IEV). IEV is transported along microtubules to reach cell periphery and fuse with plasma membrane to become cell-associated enveloped virus (CEV) that triggers actin tails on cell surfaces or is released as EEW (Fig. 17.19).