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Inside-out model for self-assembly of tobacco mosaic virus.

Incompletely reconstituted particles of tobacco mosaic virus treated with moderate concentrations of dimethylsulfoxide prior to electron microscopy have two visible tails of unencapsidated RNA, a long one and a short one. The short tail has a constant length of 720 +/- 80 nucleotides in incomplete particles of diverse size and probably corresponds to the 3'OH end of the RNA. The length of the long 5' tail is inversely related to the length of the unfinished rod. Surprisingly, both tails appear to protrude from the same end of the particle. These observations suggest that the strand of RNA coming out of one end of the particle loops back along the length of the rod so as to appear at the opposite end. The "looped-back" tail almost certainly passes down the empty central channel of the incomplete rod. The findings are compatible with a model of tobacco mosaic virus assembly in which the RNA inserts itself beneath layers of incoming protein from within the central channel.