In vivo and in vitro decline of specific infectivity of tobacco ringspot virus correlated with nucleic acid degradation.

Abstract

The specific infectivity of bottom component of tobacco ringspot virus (TRSV) declined steadily in vivo between 3 and 21 days after infection. A similar decline resulted in vitro when purified virus was incubated 1 week at 27°. This decline in specific infectivity was correlated with degradation of nucleic acids within the protein shells. Degradation of nucleic acid occurred at random, resulting in nucleic acid pieces of varying length. Both slow and fast sedimenting components (noninfectious and infectious nucleic acids, respectively) decreased relatively with the age of infection or with incubation of purified virus in vitro, and polydisperse nucleic acids increased proportionately. In vivo the decline in the specific infectivity of TRSV was accounted for additionally by the relative increase in the number of bottom component particles with two noninfectious nucleic acid strands as the age of infection increased. Decline in specific infectivity and degradation of nucleic acids occurred in vivo whether TRSV multiplied in bean or in tobacco plants. In vitro degradation of nucleic acid was not decreased in the presence of 0.04% bentonite and was not increased in the presence of 10−3 μg/ml pancreatic ribonuclease. We postulate that the in vitro degradation of TRSV nucleic acids within the protein shell is a nonenzymatic reaction and that a similar reaction may be responsible for the observed in vivo degradation of nucleic acids. The changes in nucleic acids are discussed as they relate to our hypothesis concerning the formation of top, middle, and bottom component of TRSV.