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Astroviruses, caliciviruses,
and picornaviruses have many similarities. Below are a brief summary of
the three viral families followed by some emphasis on their differences.
Caliciviruse
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ssRNA, (+) sense, infectious, linear, nonsegmented |
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icosahedral capsid morphology, naked (nonenveloped) |
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genome length is 7.8 kb (Norwalk is 7753 nt) |
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structural genes at 3' end (5' is more common) |
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subgenomic message includes 3' end and structural genes |
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fecal-oral transmission |
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180 copies of single capsid protein |
Picornavirus
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ssRNA, (+) sense, infectious, linear, nonsegmented |
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icosahedral capsid morphology, naked |
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genome length is 7.5 kb (Polio length) |
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structural proteins at 5' end |
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4 capsid proteins |
Astrovirus
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ssRNA, (+) sense, infectious |
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icosahedral capsid morphology, naked |
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genome length is 6.8 kb |
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structural proteins at 3' end |
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subgenomic message includes 3' end and structural gene |
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capsid, ORF-2, cleaved into 3 proteins |
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frameshifting (-1) involving hepatameric shift sequence A
AAA AAC |
As you can see, the three
families share many similarities. Astroviruses are distinguished from the
other families primarily by their molecular biology.
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Astroviruses have their structural
proteins at the 3' end and employ a subgenomic
message, likely to facilitate
expression of these structural proteins. This differentiates them from
picornaviruses. |
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Astroviruses employ ribosome
frameshifting. This distinguishes them from picornaviruses (which
never employ translational frameshifting) and caliciviruses (which only
sometimes employ frameshifting). |
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Astrovirus proteins differ in the
number and processing of their structural proteins, differentiating them
from calicivirus. The astrovirus structural gene ORF-2 is cleaved into
3 capsid proteins. Calicivirus has only a single capsid protein. |
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Astroviruses lack a
helicase.
This is considered unusual for positive sense RNA viruses with genomes
larger than 6 kb. |
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Finally, astroviruses have the
characteristic star-like EM appearance which distinguishes them from the
other two families.
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