The correlation between the proportions of virus-related products and the infectious component during the synthesis of tobacco ringspot virus

Abstract

A time study has suggested that at least one, of two noninfectious components found in tissues of plants infected with tobacco ringspot virus (TRSV), is synthesized concurrently with virus multiplication. Three components found in infected (but not in uninfected) tissues are referred to as top, middle, and bottom components, according to their relative positions after density gradient centrifugation. In purified preparations of a WS-strain from leaves collected 2 1 2 –3 days after inoculation, the amount of middle as compared to the amounts of middle plus bottom components combined varied between 39 and 58% (mean: 46%), based upon ultraviolet absorption at 260 mμ. On the other hand, the amount of middle component in preparations from leaves collected 6 or 7 days after inoculation varied between 17 and 38% of the mixture (mean: 25%). Although the middle component was a smaller proportion of the mixture at 6 or 7 days than at 2 1 2 or 3 days, it had actually increased an average of threefold, while the bottom component (the infectious particle) had increased an average of eightfold. Thus, between 0 and 3 days the middle component increased more rapidly in relation to the bottom component than between 3 and 7 days, when the middle component increased more slowly than the bottom component. In older infections (14 and 21 days after inoculation), the middle component either was a smaller portion of the mixture, or remained approximately the same portion. The direction of the shift in the proportion of noninfectious and infectious particles with time after inoculation was basically the same for a strain of TRSV (ST) that contained, upon purification, a much smaller proportion of middle component than TRSV (WS). Both middle and bottom components of the ST strain increased at a much slower rate than the corresponding middle and bottom components of the WS strain. Our data are consistent with the view that the noninfectious middle component is a product intimately connected with the synthesis of tobacco ringspot virus. The middle component does not appear to be the result of degradation from infectious particles either in vivo or during the course of purification. However, the middle component is not necessarily a precursor of the bottom component. It appears more likely that the middle component is synthesized concurrently with the infectious bottom component and by means of the same pool of protein and nucleic acid subunits.