The multiplication of four strains of tobacco mosaic virus was compared at temperatures of 20° to 25° and at 35° by estimating the concentration of total infective virus RNA, intact virus, virus antigen and insoluble virus protein in plants at different times after inoculation. The four strains were: the type strain, nitrous acid mutants 2 and i118, and the ‘thermophilic’ strain . The concentration of total infective RNA of all four strains reached its maximum concentration about a week after inoculation and was about ten times greater at 20° than at 35°, but the infectivity of intact virus and the virus antigen titre varied with the strain. The intact virus and virus antigen concentrations of the type strain were reduced similarly to the RNA concentration when the temperature was raised. i118 produced very little infective intact virus at 35° but as much at 20° as the type strain, although the particles were less well-formed. No intact virus of 2 was found at any temperature as the virus protein is non-functional. The strain produced about as much infective intact virus as 35° as the type strain at that temperature but much less virus at 20° than at 35°.

Virus multiplication of the four strains was affected by increased temperature in two ways: (1) the replication of the RNA of all strains was inhibited; (2) the RNA of those strains which had defective protein was degraded. Degradation was most obvious with 2, the infectivity of which, after a maximum was reached, declined at increasing rates with increasing temperature. Free RNA of i118 accumulated at 35° because the protein became insoluble but, in contrast to 2, some complete virus was produced. There was no free RNA in plants infected with type strain. The apparently greater infectivity of at 35° than at 20° resulted from the fact that its RNA is badly coated at 20°, forming unstable particles. The concentrations of total infective RNA of type strain and at 35° did not differ.

When infected plants were transferred from 35° to 20°, the infectivity of intact virus increased with the type strain and still more with i118.


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