THE PROTEINS OF MICRO-ORGANISMS 331 



plant does not exceed about 30,000. It is inactivated ]>y 

 treatment with hydrogen peroxide, formaldehyde, nitrous 

 acid or ultra-violet light, and can then no longer provoke 

 the disease nor call forth the production of further 

 protein ; the protein is not denatured, and the molecular 

 weight and crystalline form are not altered by this treat- 

 ment, nor is the serological behaviour with antisera 

 prepared against the active protein or the juice of infected 

 plants. Denaturation by acid, alkali, heat or oxidation 

 causes not only loss of activity but also loss of the other 

 characteristic properties of the protein. Covering of up 

 to 70 per cent, of the amino groups by acetylation or 

 conversion to the phenylureido group does not cause loss 

 of activity a) chough further treatment results in inactiva- 

 tion of the virus. Twenty to forty per cent, of the 

 phenolic groups of the tyrosine residues can also be 

 masked by acetylation without destroying the activity 

 of the virus. Inoculation of the acetyl- or phenylureido - 

 virus into tobacco plants gives rise to the disease and to 

 reproduction of normal virus and not acetylated virus. 

 A similar crystalline protein having the proj)erties of 

 the aucuba mosaic viiiis has also been isolated from 

 the juice of infected plants. It differs from the ordinary 

 mosaic virus protein in having larger crystals (0-03 mm. 

 long), an isoelectric point at pK 3-7 instead of 3-3, in 

 being considerably less soluble and in having a sedi- 

 mentation constant about 20 per cent, greater. The 

 virus particles are believed to be thread-like macro - 



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molecules about 3000 A units long and 150 A units wide, 

 but their size varies with the treatment used during 

 isolation, suggesting that they are built up by polymerisa- 

 tion of smaller molecules of molecular weight about 



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15,000 and 150 A units long and 15 A wide. It has been 

 suggested that the virus protein may be formed either by 

 polymerisation of the normal plant proteins or by direct 

 synthesis under the autocatalytic influence of the protein 

 itself. 



