276 ORIGIN OF STRUCTURES AND FUNCTIONS 



determining the numbers of terminal groups of the poly- 

 peptide chains. It has thus been established that the amino 

 acid at the carboxyl end of the chain is threonine. ^^^ 



All these data taken together indicate that the protein 

 component of the particle of tobacco mosaic virus consists 

 of about 2,800 separate peptide chains each of which has a 

 threonine residue at one end and is composed of up to 150 

 amino acid residues. These polypeptide chains are folded in 

 a definite way to form the fundamental molecule of the 

 protein of the virus. 



Proteins of this kind are not only to be obtained from the 

 nucleoproteins of the virus, they may also be isolated from 

 the juices of the infected plant, where they exist in the free 

 state, not combined with nucleic acid. This was done by 

 W. N. Takahashi and M. Ishii."^ These authors isolated, 

 by electrophoresis, the same protein from the tissues of 

 tobacco, tomato and phlox plants infected with tobacco 

 mosaic virus and called it X-protein. It was found to be 

 different from the proteins of the plants in question, but to 

 correspond both chemically and serologically with the pro- 

 tein of the virus. In neutral solution the particles of this 

 protein are nearly the same size as the fundamental mole- 

 cules of the virus proteins but in an acid medium they join 

 together to give rod-shaped formations which look, under 

 the electron microscope, like virus particles and have a 

 diameter of 150 A. This protein is absent from healthy plants. 



The suggestion that the X-protein is made up of fragments 

 of normal virus particles arising in the process of disintegra- 

 tion of the plant tissues has not been confirmed. On the 

 contrary, all the data now available indicate that it is the 

 immediate product of biosynthesis which takes place in the 

 living plant in the presence of the virus. The X-protein has 

 no infectivity and, accordingly, no power of self-reproduction 

 even within the living cell. 



X-ray crystallographic studies^*^ of the virus particles have 

 shown that the axis of the cylinder or core of the rod is 

 formed by the nucleic acid while the protein is arranged 

 as a covering layer around it. The polypeptide chains of the 

 fundamental protein molecule are curled round the axis 

 of nucleic acid in such a way that the whole virus particle 



