Particles of a stable isolate of tomato spotted wilt virus (TSWV) had a density of 1.147 g/ml in sucrose and contained four major structural proteins of mol. wt.: 1, 26 × 10; 2, 52 × 10; 3, 56 × 10; and 4, 78 × 10. The RNA which was not infectious consisted of four segments of mol. wt.: 1, 3.6 × 10; 2, 2·8 × 10; 3, 2·0 × 10; and 4, 1·3 × 10. The ribonucleoprotein (RNP) of TSWV, prepared by treatment of purified virus with Nonidet P-40, could be separated into three components by sedimentation on sucrose density-gradients. All three components contained protein 1; the slowest sedimenting component also contained RNA species 1, 2 and 4, the second component contained RNA species 1 and 2 and the fastest sedimenting component contained all four RNA species. The three components were slightly infectious and infectivity was enhanced by pooling these components. The RNP, when observed under the electron microscope, appeared to consist of strands.

Enveloped particles devoid of the outer layer of projections were obtained by treating TSWV with bromelain. These subviral particles retained about 1% of their original infectivity, had a density of 1·137 g/ml in sucrose and only contained protein 1. It was concluded that this protein forms the subunit of the nucleocapsid and that TSWV, unlike the majority of enveloped viruses, does not possess an internal membrane protein.


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