The Quality of Virus as Affected by the Ambient Temperature

Dolichos enation mosaic virus is a serotype of tobacco mosaic virus infecting leguminous plants; it produces a large proportion of particles shorter than 3000 Å, the accepted length of infective tobacco mosaic virus.

A new strain of dolichos enation mosaic virus isolated from a single necrotic lesion on French bean caused local and systemic necroses in French bean plants. It produced many more defective particles than the parent strain when infected plants were kept at 20° but not at temperatures above 32°. Both strains multiplied faster at 32° than at 20°, but the effect of increasing temperature was greater with the new strain. The effect on the new strain appeared greater when virus content was assayed hy infectivity tests than when it was assayed serologically, suggesting that infectivity per unit weight of virus was also greater at 32° than at 20°. No evidence was found that leaves infected with the new strain contained free infective RNA.

The ultraviolet absorption spectra of purified preparations of the new strain produced at 20° had a greater optical density at 280 mαt than at 260 mα, whereas the parent strain had greater density at 260 mα, showing that some of the particles of the new strain were without RNA. The type of particles produced by the two strains differed when purified preparations in 0·06 m- phosphate buffer at pH 8 were subjected to analytical ultracentrifugation in sucrose gradient columns or fractionation through agar columns. When produced in plants at 20°, the new strain consisted mainly of ring-like particles, virus protein, some free RNA and very few infective particles. By contrast, the parent strain consisted mainly of infective virus and broken particles of various lengths, of which a particle 400 Å long was plentiful enough to give a peak in the analytical centrifuge and a zone in sucrose gradient columns. The new strain produced in plants at 32 to 36° did not differ in appearance or infectivity from the parent strain. The distribution of particle lengths in sprayed droplets, measured in the electron microscope, confirmed the difference found by other means between the type of particle in the two strains.

Changing the pH value of a purified preparation of the new strain (produced at 20°) from 5·2 to 8 released some RNA in amounts suggesting that about a third of the particles in the preparation released their RNA.