Double-helical RNA in satellite tobacco mosaic virus

SATELLITE tobacco mosaic virus (STMV) is the spherical satellite to an obligatory rod-shaped helper tobacco mosaic virus (TMV), which is required for replication1,2. STMV has 60 protein subunits of Mr 17,500 on a T = 1 icosahedral capsid3 containing a single stranded RNA genome of 1,059 bases4–6. STMV appears similar to another virus, STNV7,8, but is ~20 per cent smaller. It shows no amino-acid homology or immunological cross-reactivity with either STNV or its host TMV4,9. Here we report the X-ray crystal structure of STMV, which shows that the coat protein of STMV contains a ‘Swiss roll’ β-barrel. An amino-terminal strand extends more than 60A and is primarily responsible for quaternary interactions. Each capsid dimer is associated with a segment of genomic RNA double helix comprising seven base pairs. The dyad of each protein dimer is coincident with that of the central base pair of the associated RNA segment whose helix axis is directed along an icosahedral edge. Protein-nucleic acid interactions are extensive. The RNA helices, which have additional stacked bases at their 3′ termini, differ significantly from canonical nucleic acid helical forms.