The differential scanning calorimetry (DSC) 'melting curves' for virions and coat proteins (CP) of wild-type tobacco mosaic virus (strain UI) and for its CP ts mutant ts21-66 were measured. Strain UI and ts21-66 mutant (two amino acid substitutions in CP: I21 ~ T and D66 ~ G) differ in the type of symptoms they induce on some host plants. It was observed that CP subunits of both U1 and ts21-66 at pH 8.0, in the form of small (3-4S) aggregates, possess much lower thermal stability than in the virions. Assembly into the virus particles resulted in a DSC melting temperature increase from 41 to 72°C for UI and from 38 to 72°C for ts21-66 CP. In the RNA-free helical viruslike protein assemblies UI and ts21-66 CP subunits had a thermal stability intermediate between those in 3--4S aggregates and in the virions, ts21-66 helical protein displayed a somewhat lower thermal stability than U1.
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