The two RNA species of tobacco ringspot virus (TRSV) had about the same radioactivity per molecule when iodinated by treatment with NaI and chloramine T. The radioactivity was not separated from the RNA by heating for 1 min at 70 °C in 100% formamide followed by rate zonal sedimentation in sucrose density gradients containing SDS, or by equilibrium centrifugation in caesium trichloroacetate, indicating that the labelled material is covalently linked to the RNA; these treatments had little effect on infectivity. Incubation with Pronase or proteinase K made 90 to 97% of the radioactivity soluble in 70% ethanol, but did not noticeably alter the size of the RNA molecules. Polyacrylamide gel electrophoresis of acetone-precipitable material, liberated from the RNA by ribonuclease treatment, revealed a radioactive component with the mobility of a polypeptide of mol. wt. about 4000. The protease-sensitive structure previously shown to be needed for infectivity of TRSV-RNA probably is the covalently linked polypeptide now detected.


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