Biological and physico-chemical properties of an atypical mutant of tobacco mosaic virus

Summary A mutant of tobacco mosaic virus (TMV) was isolated from a population of TMV-RNA molecules inactivated to 0.1% level of survival by treatment with nitrous acid This mutant is poorly transported in Samsun tobacco and induces chlorotic spots, oak-leaf patterns, ringspots and necrosis. Rate zonal sucrose density-gradient and equilibrium sedimentations in sucrose and CsCl indicate that the purified mutant preparations contain a heterogenous population of particles including some free coat protein. These vary in length, but approximately 5–10% are infectious rods of standard (15×300 nm) dimensions. Purified mutant RNA contain an overabundance of fragmented and a few infectious molecules. Compared to the parent strain, the mutant capsid protein contains two additional residues of arginine and one of glycine, but lacks one residue each of glutamic acid (or glutamine), serine and threonine. These observations and other data based on temperature sensitivity, exposure to urea and serology indicate that the mutant particles are inherently unstable and break during the purification procedure.