Abstract
Plants infected with the FLAVUM or the NI-118 strain of TMV contain electron-dense cytoplasmic X-bodies (Kolehmainen, Zech & Von Wettstein, 1965; Kassanis & Milne, 1971), whereas plants infected with the type strain do not. Strain NI-118 produces insoluble coat protein, especially in infected plants that are kept at temperatures above 30° (Jockusch, 1966; Kassanis & Bastow, 1971), and it is possible that the X-bodies contain this protein. We thought that information about the nature of the X-bodies might be obtained by comparing the behaviour of the two variants of PM2 only one of which forms insoluble coat protein. PM2 was originally obtained by treating type TMV with nitrous acid and produces a defective protein unable to coat its RNA (Siegel, Zaitlin & Sehgal, 1962). The original PM2 has a soluble protein, but a variant, isolated at Rothamsted from it, from a single local lesion, produces insoluble coat protein (Kassanis & Bastow, 1971).