Molecular Biology

The Astrovirus genome is composed of plus sense, single stranded RNA that is approximately 6,800 nucleotides long, excluding the poly-A tail. Currently, three of the seven human serotypes have been fully sequenced. The full details of the replication cycle are not known, but it is suspected that replication occurs primarily in the cytoplasm, with a possible nuclear step.

The genome is composed of three open reading frames. They encode both a full genomic and a subgenomic RNA. ORF 1a is approximately 2,842 nucleotides long and overlaps with ORF 1b (approximately 1,557 nucleotides long) by 70 nucleotides. There is a (-1) ribosomal frameshifting event between the two frames. The length of ORF 2 varies according to strain, but is between 2,358 and 2,388 nucleotides long. ORF 1a is thought to encode the viral protease, ORF 1b an RNA dependent RNA polymerase, and ORF 2 non structural proteins. This is consistent with the observation that it is a generally effective strategy for viruses to encode nonstructural proteins at the 3' end and structural proteins at the 5' end.

These characteristics are sufficient to distinguish Astroviruses from similar small, round viruses such as Caliciviruses and Picornaviruses. For example, it is known that Astroviruses have differences in the size and processing of proteins, and they lack a helicase domain. In addition, Astroviruses use frameshifting and subgenomic gene messages, which do not occur in the other viruses. Based on these factors, it was appropriate to consider Astroviruses a family in their own right.

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