The molecular organization of beet necrotic yellow vein virus

Isolates of beet necrotic yellow vein virus (BNYVV) contain rod-like virus particles of four different lengths. Using electron microscopy in combination with optical diffraction and digital image processing methods, we have determined their structural organization. All particles observe the same helical symmetry, according to which the coat protein molecules (Mr approximately 21,000) follow a single-stranded right-handed helix of pitch 2.6 nm. This helix has an axial repeat of four turns, involving 49 protein subunits. The different particle lengths are apparently dictated by the four RNA species which compose the segmented BNYVV genome, with four nucleotides protected by each coat protein subunit. The spatial arrangement of BNYVV RNA--49 residues per helical turn, pitch = 2.6 nm--is virtually identical to that of tobacco mosaic virus (TMV) RNA (49 residues per turn, pitch = 2.3 nm), although the helical packing of protein (49/3 subunits per turn) and RNA-to-protein stoichiometry (3 bases per subunit) of TMV are significantly different from those of BNYVV (49/4 subunits per turn and 4 bases per subunit, respectively).