Defective isolates of tomato spotted wilt virus (TSWV) which produce amorphous masses characteristic of TSWV in infected cells but do not produce enveloped virus particles have been studied. Nucleocapsid protein but no membrane proteins was detected by ELISA in plants infected with defective isolates. The protein of an infectious fraction purified from such plants consisted mainly of nucleocapsid protein. Electron microscope examination of this purified fraction revealed structures resembling the amorphous masses observed in thin sections of leaves infected with the defective isolates. This fraction from the three defective isolates studied contained three RNA segments, the mol. wt. of RNA 1 and RNA 3 being the same as those of normal TSWV particles. The mol. wt. of RNA 2 of the defective isolates was always smaller than that of the RNA 2 of normal TSWV particles. However, their mol. wt. were different for each isolate. Thus, the amorphous masses are aggregates of nucleocapsids of defective TSWV which do not produce normal particles since they have lost the ability to make at least one membrane protein species. This is possibly due to the deletion in RNA 2.


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