Effect of 32P Decay on Infectivity of Tobacco Mosaic Virus

The amount of survival 32P-labeled tobacco mosaic virus (TMV), 6×104cpm per mg of TMV, was decreased exponentially with time and it became about 1/8 of original amout after 28 days storage. However, no reduction in amount of survival virus was noted when unlabeled TMV was mixed with 32P-orthophosphate, 6×104cpm per mg of TMV. In despite of loss in infectivity, no destruction of virus perticles could be seen in electron microscopic observation of 32P-labeled TMV after 28 days storage. The results suggest that the loss of infectivity of 32P-labeled TMV is due to transmutation (32P→32S) of the isotope incorporated in the virus particles, and not due to 32P β-radiation.