A temperature-sensitive mutant of tobacco mosaic virus deficient in synthesis of single-stranded RNA.

RNA synthesis by a temperature-sensitive, replication-deficient mutant of tobacco mosaic virus (TMV), IV-35, was examined in temperature-shift experiments. Viral RNA synthesis occurred normally at 25°, but upon a shift to 35°, synthesis of single-stranded genome RNA ceased immediately while production of replicative form and replicative intermediate RNAs continued normally. The plus strand:minus strand ratios of IV-35 double-stranded RNAs labeled at 35° were the same as those of wild type. The ability to produce single-stranded RNA was not irreversibly lost at 35° because upon a shift back to 25°, synthesis of single-stranded RNA immediately resumed. The in vitro activity of membrane-bound replicase was not temperature sensitive. However, the stabilities in vitro and in vivo at 35° and the optimum temperature profile of the enzyme from IV-35-infected leaves were altered from those of the wild-type enzyme preparation.