Seven polypeptides were found to be present in the capsid of the bluetongue virus (BTV), four of which are major and three are minor components. This number and size distribution is the same as that found in reovirus, which has a similar segmented, double-stranded ribonucleic acid genome. In both viruses an excellent correlation is found between the molecular weights of certain genome segments and those of the polypeptides, suggesting a direct coding relationship between them. Careful comparison, however. indicates that the correlation patterns of the two viruses are different, suggesting that different genome segments may be utilized to code for certain capsid proteins. This phenomenon may be related to the fact that these viruses can be distinguished morphologically, reovirus having a larger diameter and possessing a double-layered capsid against the smaller single capsid of the BTV. Utilizing the degrading effect of CsCl gradients, it was demonstrated, however, that two of the BTV polypeptides are present as a diffuse protein layer surrounding the capsid. This outer layer probably has some of the functions of the reovirus outer capsid, one being the "masking" of a viral transcriptase which could be demonstrated after its removal.

SummaryPurified African horsesickness virus was shown to possess an icosahedral shape, measure approximately 55 mix in diameter and probably consist of 32 capsomeres. Electron microscopic evidence indicates a close morphological relationship between the virus and bluetongue virions. African horsesickness virus has a double-stranded RNA genome which was resolved into five components by sucrose gradient sedimentation analysis and into six segments in four size groups by means of polyacrylamide gel electrophoresis. The remarkably close relationship between African horsesickness and bluetongue viruses has led to the suggestion that these two viruses be classified in one sub-group of the newly proposed diplornavirus group.

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