The isometric particles of lucerne transient streak virus (LTSV) contain linear ssRNA of mol. wt. approx. 1.4 × 10 (RNA-1) and ssRNA of mol. wt. approx. 1.2 × 10 in both a linear and circular form (RNA-2). Unfractionated LTSV RNA induced necrotic local lesions in leaves of whereas RNA-1, partially separated from RNA-2 by gel electrophoresis, induced many chlorotic local lesions but few necrotic ones. Cultures obtained from either lesion type continued to induce only chlorotic (C isolate) or necrotic lesions (N isolate) on subsequent passage. Apart from the lesion type in species, the isolates were indistinguishable except that particles of isolate N contained both RNA-1 and RNA-2 whereas those of isolate C contained RNA-1 but no RNA-2; RNA-2 was also not detected in leaves inoculated with isolate C. RNA-2 alone did not infect but when it was added to inocula of RNA from isolate C, a proportion of the lesions induced were necrotic and this proportion increased with increasing concentration of RNA-2. The infectivity of RNA-1 was destroyed by treatment with snake venom phosphodiesterase or proteinase K but the ability of RNA-2 to alter the lesion type induced by isolate C was unaffected by these treatments, suggesting that the molecules of RNA-2 are biologically functional and do not need a genome-linked protein for this activity. These results suggest that RNA-2 found in particles of LTSV is not part of the virus genome but may be a satellite RNA that affects symptom expression.


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