Temperature sensitive mutants of bacteriophage Qβ have been isolated which fail in the synthesis of their virus RNA at the non-permissive temperature (42 °C). Nine mutants have been studied in some detail. Cells infected with these mutants at 37 °C and incubated long enough to produce substantial amounts of Qβ RNA cease Qβ RNA replication when shifted to 42 °C. The mutants can be classified into 3 groups according to the amount of Qβ RNA replicase activity exhibited in extracts from infected cells isolated at various times after shift to 42 °C: in group 1 mutants, enzyme activity is the same, regardless of the time of isolation after shift; in group 2 mutants enzyme activity increases with time of isolation after shift; in group 3 mutants, enzyme activity decreases with time of isolation after shift. Synthesis of all virus proteins is suppressed at 42 °C in cells infected with group 1 or group 3 mutants. In cells infected with group 2 mutants, synthesis of Qβ RNA replicase subunit β is increased, but synthesis of other virus proteins is depressed at 42 °C. The inhibition of virus RNA and protein synthesis is reversible. A detailed analysis of these experiments suggests that a defective Qβ RNA replicase is involved in the inhibition of both virus RNA and protein synthesis.


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