1. Denatured tobacco mosaic virus has a number of SH groups corresponding to its total sulfur content of 0.2 per cent. The SH groups were estimated by titration with ferricyanide, tetrathionate, and p-chloromercuribenzoate in guanidine hydrochloride solution and by reduction of the uric acid reagent in urea solution.

2. The SH groups of tobacco mosaic virus or their precursors can be abolished by reaction of the native form of the virus with iodine.

3. Tobacco mosaic virus whose SH groups have been oxidized beyond the S-S stage by iodine but whose tyrosine groups have not been converted into di-iodotyrosine groups still retains its normal biological activity as shown by the number of lesions it causes on Nicotiana glutinosa plants and by the characteristic disease produced in Turkish tobacco plants.

4. The inoculation of Turkish tobacco plants with active virus whose SH groups have been abolished by iodine results in the production of virus with the normal number of SH groups.

5. If enough iodine is added to tobacco mosaic virus or if the iodine reaction is carried out at a sufficiently high temperature, then the tyrosine groups are converted into di-iodotyrosine groups and the virus is inactivated.

6. Tobacco mosaic virus can be almost completely inactivated by iodoacetamide under conditions under which iodoacetamide reacts with few if any of the protein's SH groups.

7. Tobacco mosaic virus is not inactivated by dilute p-chloromercuribenzoate.

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