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The structural results confirm the notion of divergent evolution of the capsid polyproteins of nepoviruses, comoviruses and picornaviruses from a common ancestor. A number of residues were found to be conserved among various nepoviruses, some of which stabilize the quaternary structure of the three domains in the TRSV capsid protein subunit. Two conserved regions were identified on the external surface of TRSV, however, mutational studies will be needed to understand their functional significance. Nepoviruses transmitted by the same nematode species do not share regions with similar amino acid composition on the viral surface.

Tobacco ringspot virus, a plant virus that is believed to fill an apparent niche in the evolution of picornavirus-like capsids, has been crystallized by vapor diffusion with potassium phosphate and polyethylene glycol buffered at pH 6.5 in a new crystal form. The monoclinic crystals belong to the space group C2 with unit-cell dimensions of a = 407.1, b = 399.7, c = 285.9 A and beta = 129.1 degrees and diffract synchrotron radiation to 3.3 A. One half of a virus particle constitutes the crystallographic asymmetric unit. The orientation of the virus particle in the unit cell was determined by the rotation function analysis of a partial data set that has been collected at CHESS using image plates. Development of a suitable phasing model for the high-resolution structure determination of TRSV with the real-space molecular replacement technique is now under way.

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