DNA Genome Types: |
By contrast, the smallest prokaryote genome size is about 500 kb (E coli = 4000 kb)
All viruses except Polydnaviridae have single-component genomes; the latter have multiple components ranging in size from 2 - 20 kb, and the number which constitute an individual genome is not known.
Replication of the viruses is in all cases by the semi-conservative method favoured by cellular genomes; however, smaller circular genomes (eg. Papovaviridae) replicate by means of bidirectional replication forks from a single origin, like some plasmids. Among the viruses of Eukarya, replication mainly occurs in the nucleus, using cellular enzymes such as polymerases, methylases, etc. However,the replication of Poxviruses, some Baculoviruses (granulosis group), and some of the replication of iridoviruses, takes place in virus-specified "inclusion bodies" in the cytoplasm, using viral-coded enzymes, most important of which are DNA-dependent DNA polymerases.
Virus Taxonomy: Sixth report of the International Committee on Taxonomy of Viruses (FA Murphy et al., Eds.); Springer-Verlag, Wien, 1995.
Replication of all of the viruses requires formation of a "replicative form" (RF) double- stranded DNA intermediate: this is formed soon after infection, almost certainly by the host cell DNA polymerases engaging in "repair" of the ssDNA.
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The replication machinery of geminiviruses
and nanoviruses from plants, circoviruses,
anelloviruses and even parvoviruses from animals, and the
aforementioned phages and even plasmids, all has a
common origin - which may extend to the mobilisation mechanism used
by bacteria like Agrobacterium tumefaciens. A general scheme
for ssDNA virus replication can be seen
here. Geminiviruses, like other ssDNA entities, have a rep gene, producing a Rep protein: in plants, this is expressed from a double-stranded replicative intermediate form of the genome (RF), and cleaves the genome (+) strand at a specific ori sequence, binds to the free 5′ end, and then mediates ligation of the newly-displaced (+) strand. The accumulation of ssDNA seems to depend on the production of coat protein, which probably sequesters nascent ss(+)DNA, as CP- mutants produce no ssDNA. There is a fair degree of specificity in all this, with viruses having a specified host range, and Reps having specificity for a narrow range of virus genomes.
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In the case of circular genomes in Eukarya (circoviruses, eg. porcine circovirus; also psittacine beak-and-feather disease agent, chicken infectious anaemia agent; Geminiviridae; BBTRV), these get converted into cccds-(plasmid-like)-DNA in the nucleus, and become associated with nuclear proteins and complexes such as nucleosomes.
Parvoviruses have an interesting strategy for replicating their genomes, which uses internal or self-complementarity of genome ends to get around the problem of how to replicate a linear DNA genome. A virus-specific process is required both to nick RF DNA, and to sequester newly-formed genomic ssDNA into assembling particles: in the case of Parvoviridae the first is done by a NS1 protein (which binds the new 5'-terminus resulting from the nick) and the second by the coat protein; in Geminiviridae it appears as if the first is done by a similarly-acting Rep protein, and the second also by the coat protein.
Parvoviridae: Fields Virology (2nd Edn), Chapter 62
Virus Taxonomy: Sixth report of the International Committee on Taxonomy of Viruses (FA Murphy et al., Eds.); Springer-Verlag, Wien, 1995.
Copyright Ed Rybicki, November 1997, August 1998, March 1999, October 2000, May 2008
