Chemical modification of viral ribonucleic acid: II. Bromination and iodination

• Published in: Biochimica et biophysica acta. Specialized section on nucleic acids and related subjects Vol. 72; pp. 217 - 229
• Main Author: Brammer, K.W.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 1963
• ISSN: 0926-6550; 1878-2256
• DOI: 10.1016/0926-6550(63)90335-9

The halogenation of tobacco mosaic virus (TMV)-RNA was studied using low levels (molar ratio of reagent: nucleotide was 1:5 and less) of brominating and iodinating agents under conditions in which infectious TMV-RNA itself was stable. The site of reaction in RNA were the bases, of which only adenine was completely resistant to halogenation. N-Bromosuccinimide reacted with both guanine and cytosine at pH 7 and pH 9 although reaction with guanine was more favored at pH 9. Bromine showed a more specific action, reacting almost exclusively with cytosine at pH 7 and with guanine at pH 9. Although free uridylic acid was readily brominated by both reagents, the uracil residues in RNA were apparently less reactive than either guanine or cytosine. Consequently, very little, if any, bromination of uracil was detected in RNA under the conditions used. The loss of infectivity of TMV-RNA upon reaction with bromine or N-bromosuccinimide indicated that both reagents had a high efficiency of reaction. Using similar mild conditions, iodination of TMV-RNA was not achieved with iodine in potassium iodide solution. N-Iodosuccinimide, although about ten times less efficient than the brominating agents, did inactivate infectious TMV-RNA at pH 7 and 0°. Cytosine was the most readily iodinated base under these conditions. Using the reagent labeled with 131I it was found that the binding of 2–3 atoms of iodine per infectious RNA molecule was lethal. The bromination and iodination of guanylic acid and cytidylic acid with these reagents under the conditions used for the halogenation of TMV-RNA provided some evidence regarding the probable nature of the chemical changes which occurred in the macromolecule. Some aspects of these reactions in relation to the mutation of TMV are discussed.