The secondary structure of RNA in particles of two plant viruses with helical symmetry (tobacco mosaic virus (TMV) and potato virus X (PVX)) was studied by determining their RNA extinction spectra . The true values of extinction of light-scattering virus suspensions were calculated by extrapolation. To determine the degree of hyperchromicity of internal RNA, the spectra of suspensions of intact and disrupted TMV and PVX were compared. Virus particles were disrupted by heating or by sodium dodecyl sulphate. The contribution of protein to the total extinction of virus was taken into account during calculations of hyperchromicity of internal RNA. It was shown that RNA in particles of both viruses is in a fully hyperchromic state. The interaction of both internal RNA's with formaldehyde was also investigated. The reaction of RNA with formal-dehyde was followed by the increase in extinction of the virus suspensions in the 280 to 290 nm. spectral region. It was found the amino-groups of the bases of internal PVX RNA readily react with formaldehyde. On the other hand, the TMV RNA was completely resistant to formaldehyde. A substantial difference in the optical rotatory dispersion (ORD) curves of two viruses was also revealed. The differences in reactivity with formaldehyde and in ORD curves of TMV and PVX probably reflect the different character of RNA-protein interactions in particles of the two viruses.


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