Tobacco mosaic virus, amino acid code

For instance, if such a substitution is induced in the tobacco mosaic virus genome, changes in the viral protein can be detected, which correlate with the mutation. This has been made possible because the viral protein has been characterized. Thus, it is found that the amino acid threonine is replaced by isoleucine. It is also clear that several triplet codes in the nucleic acid specify the amino acid threonine, namely AC A, AGG, ACC, and ACU, emphasizing the degeneracy of the code. 

Nitrous acid-induced mutations in tobacco mosaic virus (TMV) have contributed to the derivation of the genetic code (see Table 8.1) through amino acid analyses determining replacements in the coat protein (2). Since only C- U, G- A or A->G transitions are possible in the case of a nitrous acid point mutation, the number of possible replacements for a given amino acid is small. Thus, an Ala Val transition corresponds to a change of the codon GCN to GUN, whereas an Ala- Thr transition must be due to a change in the first letter GCN- ACN (See Chapter 8, Table 8.1). 

Wittman (1963) studied 133 mutants of tobacco mosaic virus (117 obtained by the action of nitric acid and 16 spontaneous mutants) in this respect. Of the 117 nitric acid mutants, 82 had no change in the protein particle of the coat, and only 35 had various substitutions in the amino acid chain. These facts are regarded as proof that only about one-third of the polynucleotide particle of tobacco mosaic virus RNA codes the protein subunits of the virus coat... 

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