The effect of tobacco mosaic virus biosynthesis on the nucleic acid content of tobacco leaf

The nucleic acid contents of various fractions of otherwise identical uninfected and TMV-infected tobacco leaf tissue cultured in nutrient were determined at intervals after inoculation. The effects of infection are restricted to the buffer-insoluble portion of leaf homogenates. Prior to the appearance of free TMV, the nucleic acid of this fraction in infected tissue exceeds the comparable value in uninfected tissue by an amount which is slightly in excess of the nucleic acid later found in the maximum amount of TMV formed by the tissue. As free TMV appears, this excess disappears and a deficiency in nucleic acid content develops. At the end of the infection process, the difference due to infection is nearly zero. When these effects are analyzed in terms of the separate nitrogen bases it is found that the excesses in uracil, cytosine, and adenine which precede the appearance of TMV are nearly equal to the amounts of these bases finally found in virus nucleic acid. On the other hand, the guanine excess is about twice that required for TMV formation. The results suggest that the excess nucleic acid which accumulates before TMV appears represents a precursor of TMV nucleic acid, but that other processes, including degradation of host nucleic acid are also involved in virus replication.