RNA synthesis induced by a temperature-sensitive RNA negative mutant, ts-4, of Semliki Forest virus was studied both at the permissive (28 °C) and restrictive temperature (39 °C). At 28 °C ts-4 directed RNA synthesis was qualitatively similar to the wild type but overall synthesis remained somewhat lower. Only about 3% of RNA was synthesized compared to the wild type virus when ts-4 was maintained at 39 °C throughout the infection. The sedimenting radioactivity was almost exclusively in 42S RNA. When ts-4 infected cultures were shifted from 28 to 39 °C at 5 h post-infection, the synthesis of 26S RNA was shut off while 42S RNA synthesis continued. The synthesis of 26S RNA started again even in the presence of cycloheximide if the cultures were shifted back to 28 °C. Our results suggest that 26S RNA synthesis is controlled by a virus-specific protein, the function of which is not required for the synthesis of 42S RNA, In ts-4 this protein is denatured at 39 °C and becomes non-functional, but renatures and becomes functional upon shift down to 28 °C.


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