Fig. 14.6
From: Viruses with Single-Stranded, Positive-Sense RNA Genomes
Genome organization of astroviruses. The genome of astroviruses contains three open reading frames. The 3′ and 5′ ends of ORF1a and ORF1b overlap. A hairpin structure induces a translational frameshift leading to an in-frame fusion of ORF1a and ORF1b. The precursor protein NSP1ab contains, in addition to a protease (3CLpro), also an RNA-dependent RNA polymerase. This enzyme is responsible for the synthesis of the negative-sense RNA molecules, from which the positive-sense RNA genomes of progeny viruses are transcribed. ORF2 encodes the precursor protein of structural proteins. It is cleaved by the activity of a trypsin-like protease as a part of the clearage product NSP1b