Entry into the Cell and Uncoating

• Vaccinia Life Cycle by TK Kenyon, 2009
  
  The outer viral membrane of the vaccinia virus fuses with the host cell's plasma membrane, releasing the virus into the cell's cytoplasm.
  
  Following release, the virus undergoes a second uncoating step to release nucleoproteins and the DNA. Within minutes after infection, viral enzymes that enter the cell with the vaccinia virus cores initiate transcription, the copying of messenger RNA from the DNA viral chromosome.
  
  Host DNA, RNA and protein synthesis are switched off almost immediately by viral factors carried into the cell with the virion.
  
  

Early Transcription and Early Proteins

• Early viral transcription is transcribed by a multi-subunit viral RNA polymerase. About one-half the genome or 100 genes are transcribed before the DNA is replicated.
  
  One major protein produced before DNA replication is the viral DNA polymerase. See the DNA Replication section for more information.
  
  Another early protein is a viral thymidine kinase (TK.) Surprisingly, the poxvirus TK molecular weight is about 20Kd, which is approximately half the weight of TKs from prokaryotes, eukaryotes or herpes viruses. Moreover, no sequence homologies between poxvirus TK and herpesvirus TK were found, suggesting the two TKs arose from independent evolutionary paths.
  
  Early viral mRNAs are translated on cellular ribosomes soon after they are transcribed.
  
  

Vaccinia Virus DNA Replication

• The DNA of poxviruses (a linear, double-stranded, DNA genome) is replicated in the cytoplasm of the host cell, a feature unique among viruses. All other families of eukaryotic viruses must infiltrate the nucleus of the host cell to replicate their DNA.
  
  The vaccinia virus DNA polymerase not only synthesizes a new strand, but can fill gaps and has a 3' exonuclease activity, meaning it likely has a proofreading function. Most proteins involved in replicating vaccinia virus DNA are probably virus-encoded, meaning that the virus needs few cellular enzymes to produce a new viral chromosome.
  
  

Late Transcription and Late Proteins

• There are two classes of proteins produces in late vaccinia virus infection: an immediate-late class transcribed directly after DNA synthesis and a delayed-late class.
  
  Proteins produced during both these stages include most of the structural proteins of the virus particles, some of which are processed or cleaved during virion assembly, as well as certain viral enzymes that turn off early viral protein synthesis and lock the infection into virion production mode.
  
  

Virus Assembly, Maturation and Release

• First, virions form in circumscribed and granular areas in the cytoplasm that appear dark when viewed with an electron microscope. Lipid crescents (or cup-like structures in three dimensions) are seen. These cups surround granular genome material.
  
  Then, immature viral particles appear circular or spherical in three dimensions. Ribosomes are not present near the sites of virion assembly, so the proteins and DNA genomes are transported around the cell to these sites.
  
  Mature vaccinia virus virions are moved out of assembly areas and transported to the cell membrane. Mature virions are drawn to the tips of specialized, actin-containing microvilli from which they are released.
  
  The double-layer membrane surrounding virions is thought to be derived from the Golgi apparatus. Seven viral glycoproteins stud the viral envelope.