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Some investigators of 1950s focused their attention on viruses containing protein and RNA, but no DNA. The poliomyelitis virus, influenza virus and the virus that causes mosaic disease in tobacco plants (TMV) were thus shown to have genes made of RNA.
Earlier in 1935 W. M. Stanley had reported the isolation of a crystalline protein from infected tobacco plants, which possessed the properties of tobacco mosaic virus. Continuing this work in 1950’s, H. Fraenkel-Conrat and his colleagues at Berkeley, California found that Stanley’s crystals in fact consisted of ribonucleoprotein—a compound of protein and RNA. They designed experiments to find out whether the genes of this virus were made of RNA or protein or both.
They separated the protein and RNA of the virus and inoculated them into healthy tobacco plants. However, to find out if the genes of TMV are made of RNA or protein, Fraenkel-Conrat and Singer carried out further experiments. The virus has a number of genetically different strains which can be distinguished from the type of symptoms they produce in infected leaves, and also from their differing contents of amino acids in proteins.
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For example, strain HR contains the amino acids histidine and methionine, both of which are absent from the standard TMV strain. They took these two strains and separated their protein and RNA. Then they reformed hybrid viruses by combining the protein of one strain with the RNA of the other strain. The hybrid viruses were made to infect healthy tobacco plants where they replicated to produce new viruses.
The types of symptoms produced were also analysed. The new viruses had protein coats identical to the original virus from which RNA had been isolated. In each case the symptoms were identical to those of the strain from which RNA had been isolated to reconstitute the hybrid virus (Fig. 13.3).
The same results were obtained with various combinations of nucleic acid from 4 different strains and of protein from 3 strains. The conclusion was obvious that genes of RNA viruses are made of RNA and not protein.
