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The multiplication of the DG strain of cowpea severe mosaic virus (CPsMV-DG) was studied in cowpea protoplasts prepared from infected leaves and in protoplasts inoculated in vitro. Up to six proteins were detected in DG-infected cells which were either absent in mock-infected protoplasts or present in smaller amounts. Their mol. wt. were estimated to be 125000, 98000, 86000, 65000, 39000 and 22000. The latter two probably represent the two viral capsid proteins. The 125000 mol. wt. protein was found early in infection and was synthesized at a high rate throughout infection. It was detected, together with the 86000 mol. wt. protein, in protoplasts that had been inoculated with purified bottom component alone.
CPsMV was also translated in a messenger-dependent reticulocyte lysate. Total DG strain RNA directed the synthesis of polypeptides with mol. wt. of 200000, 125000, 108000, 98000 and 42000. No cleavage of the in vitro products was observed by varying the temperature or the concentration of dithiothreitol (DTT) in the translation mixture. The polypeptides induced by CPsMV-DG were compared with those induced by the SB strain of cowpea mosaic virus. Since the 170000 and 30000 mol. wt. proteins typical of the SB strain were not detected among the DG-specific proteins either in vivo or in vitro, a different processing of the large precursor of 200000 mol. wt. seems to take place.
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