The infectivity and inactivation of nucleic acid preparations from tobacco mosaic virus

The infectivity of nucleic acid preparations made by disrupting tobacco mosaic virus with phenol was increased, relative to that of intact virus, by keeping test plants in darkness or at 37° for some time before they were inoculated. The differences in susceptibility to infection of leaves in different physiological states was too great to be explained by differences in the ability of leaf extracts to inactivate nucleic acid preparations in vitro. The spontaneous inactivation of the preparations in vitro was not prevented by inhibitors of ribonucleases and most additions to the preparations increased the rate of inactivation. Not all the inactivations are readily explicable on the assumption that the minimal infective unit is a pure nucleic acid built up solely from nucleotides. Leaf sap and saliva are reasonably assumed to inactivate because they contain ribonuclease; also, inactivations by formaldehyde phenylglyoxal, and thiaminase in the presence of thiamine, may well reflect reactions with known components of nucleic acids. However, it is less easy to invoke such actions to explain the inactivation by spermine, ‘interferon’, some oxidizing agents, leaf mitochondria in the presence of some other substances, and the greater rate of inactivation in vacuo than in air. Although nucleic acid seems essential for infectivity, it seems prudent to suspend judgement about the precise chemical identity of the minimal infective unit.