Biology Reference
In-Depth Information
Fig. 1.
Genomic organization of HAstV. Genomic and subgenomic RNA
molecules are indicated in the upper panel, and ORFs with their main func-
tional motifs are shown in rectangles. Ribosomal frame-shifting signal (RFS)
between ORF1a and ORF1b is indicated. The presence of a VPg protein at
the 5' end of RNA has not been confirmed. Nucleotide and amino acid posi-
tions are numbered according to HAstV-1 Oxford reference strain (accession
no. L23513).
replication when transfected with
in vitro
transcribed RNA, and virions
produced in these cells can infect CaCo-2 cell monolayers in trypsin-
containing media.
Virions of HAstV contain a positive sense single-stranded 6.8 kb
RNA genome, which is polyadenylated at the 3'-end and includes a
5'-untranslated region (UTR), three overlapping open reading frames
(ORFs), and a 3'-UTR (Fig. 1). Although suggested, the presence of
a covalently linked VPg protein at the 5'-end of the genome has not
yet been biologically demonstrated.
17
The fact that
in vitro
tran-
scribed capped RNA is infectious does not rule out the possibility that
the 5' end of genomic RNA found within particles could be linked to
a VPg protein, and similar observations have been made with feline
calicivirus.
18
ORF1a and ORF1b are linked by a ribosomal frame-
shifting signal (RFS) and code for the nonstructural proteins, includ-
ing a serine protease and an RNA-dependent RNA polymerase, while
ORF2 encodes the capsid precursor.
11
Although it is considered characteristic of positive-stranded RNA
viruses with genomes larger that 6 kb to encode a helicase domain,
there was a widespread belief for many years that astroviruses did not
code for a helicase protein. However, two putative helicase conserved
