SUMMARY: An inhibitor of plant viruses can be isolated from the sap of by differential precipitation with ethanol followed by adsorption on celite and elution with 10% NaCl. Purified preparations contain 14--15% nitrogen and 8--12% carbohydrate and the inhibitor is probably a glycoprotein. Denaturation leads to loss of inhibiting power. The protein, unless denatured, is unaffected by pepsin and trypsin.

The glycoprotein is isoelectric at about pH 7. It can combine with tobacco mosaic virus, and when salt-free solutions of the two are mixed in certain proportions at pH values between their isoelectric points it precipitates the virus in the form of paracrystalline threads. The glycoprotein also precipitates tomato bushy stunt virus.

When added to several plant viruses, the glycoprotein causes an immediate reduction in infectivity, but has no effect on a bacteriophage. Non-infective mixtures regain infectivity when diluted. No evidence was found for a combining ratio of virus to inhibitor necessary to cause loss of infectivity. The mechanism of virus neutralization is discussed.


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