Microviridae: Microviruses and Gokushoviruses

James E Cherwa, University of Arizona, Tucson, Arizona, USA
Bentley A Fane, University of Arizona, Tucson, Arizona, USA

Published online: May 2011

DOI: 10.1002/9780470015902.a0000781.pub2

Members of the Microviridae comprise two subfamilies. The microviruses (Greek for small), which infect free-living bacteria, are among the fastest known replicating viruses. Gokushoviruses (Japanese for very small) occupy a unique niche, infecting obligate intracellular bacteria, such as Chlamydia and Bdellovibrio, or mollicutes, bacteria without a cell wall. All members of the family contain small (4000–6000 bases), circular, single-stranded deoxyribonucleic acid (ssDNA) genomes of positive polarity, which are packaged inside small (~25 nm diameter) T=1 icosahedral capsids. The other icosahedral, ssDNA virus families: Parvoviridae, Circoviridae, Nanoviridae and Geminiviridae; share most of these properties, suggesting a large super family spanning several domains of life. The most well known member of the Microviridae, X174, has been extensively used to study the fundamental mechanisms of DNA replication and capsid assembly. The latter is uniquely dependent on two scaffolding proteins, and has become a model system for experimental evolution.

Key Concepts:

  • Whilst overlapping reading frames increase the amount of genetic information encoded in small genomes, they do not appear to significantly impact the ability of the virus to genetically adapt to selective pressures.
  • Due to the genome's positive polarity, DNA replication must commence before viral genes can be transcribed.
  • Microvirus DNA replication occurs in three distinct stages: (1) ssDNA is first converted to a double-stranded molecule, (2) amplification of the double-stranded molecule, (3) single-stranded genomic DNA synthesis and packaging.
  • Genomic DNA synthesis and packaging are concurrent processes; thus, a genome is not synthesised unless there exists a capsid in which to package it.
  • Gene expression is controlled by the finely tuned interplay of cis-acting genetic elements: promoters, ribosome binding sites, mRNA stability sequences and transcription terminators.
  • Microviruses are distinguished by their two scaffolding protein system, whereas Gokushoviruses utilise a single scaffolding protein.
  • Capsid assembly is mediated by scaffolding proteins, which induce conformational switches in the viral coat protein to control the timing and fidelity of morphogenesis.
  • Cell lysis is achieved by inhibiting host cell wall biosynthesis, a mechanism reminiscent of some antibiotics.

Keywords: bacteriophage; icosahedral; ssDNA; atomic structure; scaffolding protein; procapsid

Figure 1. Virion and procapsids structures. (a) Atomic structure of X174, a microvirus. (b) CryoEM image reconstruction of SpV4, a gokushovirus. (c) CryoEM image reconstruction of the X174 procapsid. The external scaffolding protein is shaded in light green, the major spike protein in blue, and the viral coat protein in magenta. The triangle highlights an asymmetric unit, which contains one coat, one major spike and four external scaffolding proteins. The oval, pentagon and triangle depict the respective two-, five-, and three-fold axes of symmetry. (d) The atomic structure of the four D proteins found in the asymmetric unit. Subunits D1, D2, D3 and D4 are depicted in green, yellow, red and turquoise, respectively.
Figure 2. Genetic maps. (a) Linear depictions of microvirus and gokushovirus genetic maps. With the exception of the HA intercistronic inset, maps are drawn to scale. For SpV4, the genes corresponding to the MH2K and Chp2 genes are given in parentheses. (b) Circular depiction of the Microvirus genetic map. (c) Transcription map of X174. Promoters and terminators are depicted with ‘P's’ and ‘T's,’ respectively. The subscript letter demarks the location and name of the promoter or terminator. Grey lines represent transcripts. The width of the line reflects the gene transcript's relative abundance in an infected cell.
Figure 3. The X174 lifecycle. The roles and functions of the viral proteins are listed in Table 1. The host cell rep protein is required for proper DNA packaging.
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Related Sub-Topics:

DNA Viruses | Virus Structure | Viruses Infecting Bacteria
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Article Title: Microviridae: Microviruses and Gokushoviruses
 
How to Cite close
Cherwa, James E, and Fane, Bentley A(May 2011) Microviridae: Microviruses and Gokushoviruses. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000781.pub2]