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Comparisons of the coat protein sequences of four tobraviruses with those of seven tobamoviruses indicate that these proteins share a common evolutionary origin. Numerous amino acids for which specific functions have been identified in the molecular structure of the tobacco mosaic virus vulgare protein have identical or closely similar counterparts among the tobraviral proteins. These include those with roles in the hydrophobic core of the protein, those that contribute to the RNA binding site and those involved in the control of virus assembly. We suggest a model for the structure of the tobraviral particle that not only offers an explanation for the greater diameter of the tobraviral particle but also confirms an early suggestion for RNA placement within this particle.