Biological effects of the incorporation of thiouracil into the ribonucleic acid of tobacco mosaic virus

Abstract

1. 1. Tobacco mosaic virus, grown in the presence of thiouracil contains per unit weight a reduced quantity of material susceptible of forming new virus after infecting Nicotiana tabacum. This reduction in the quantity of infectious material appears to be related to the incorporation of thiouracil into the virus nucleic acid. If uracil is added to the culture, at the same time as thiouracil, the incorporation of the latter in the nucleic acid is considerably diminished and the quantity of infectious material contained in the virus is normal. The protein part of the virus incorporates a small quantity of the S from the thiouracil. This incorporation is without effect on virus infectivity. 2. 2. On the other hand the incorporation of thiouracil in virus nucleic acid does not modify appreciably the number of particles per unit weight of virus that are capable of producing a necrotic lesion in Nicotiana glutinosa. This incorporation is thus capable of diminishing the average quantity of infectious material contained in each particle without necessarily suppressing the infectivity of the whole particle. Each virus particle appears thus to contain several series of specific structures responsible for the transmission of the genetic characters of the virus. 3. 3. When virus is cultivated in the presence of thiouracil, uracil being added after a few hours, the growth of the virus appears strongly inhibited when compared to a control where thiouracil and uracil had been added simultaneously at the beginning of the culture. The inhibitory effect of the incorporation of thiouracil into nucleic acid is sufficiently important to be considered as the major cause of the inhibitory action of thiouracil on the growth of the virus. Thiouracil does not appear to act appreciably as a competitive inhibitor of a synthetic mechanism indispensable for virus growth.