To determine the structural requirements for cucumber mosaic virus (CMV) satellites to elicit lethal tomato necrosis, three satellite variants D, S and Y were used in the construction and cloning of chimeric cDNAs. D and S are necrogenic and non-necrogenic ‘prototype’ variants, respectively, and Y possesses the 3′ conserved necrosis-determining region but does not cause lethal tomato necrosis. Its 5′ half harbours an insertion/deletion region that results in a molecule about 30 nucleotides longer than other variants. Tomato bioassays were conducted with RNA transcripts of all six chimeric combinations of the 5′ and 3′ halves of the three satellite variants divided by a common restriction site, as well as with a mutated chimera. None of the chimeras containing the 5′ half of Y induced lethal necrosis in tomato even when their 3′ halves were that of the D variant with the conserved necrogenic element. Chimeras with the 3′ half of Y elicited only partial or restricted necrosis which was much less severe than that induced by prototype variant D, and often was not lethal. Site-directed mutation of a single nucleotide in proximity to the necrogenic element of such a chimera containing the 3′ half of Y restored much lethal necrogenicity. The results revealed the presence of structural elements in CMV satellite variant Y that modulate or even suppress the expression of the 3′ conserved necrosis-determining element. They indicate that in CMV satellites widely separated sequence elements constituting a three-dimensional requirement are responsible for eliciting lethal necrosis in tomato.


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