The effect of loss of regulation of minus-strand RNA synthesis on sindbis virus replication

During the replication cycle of Sindbis virus minus-strand synthesis stops normally at the time that plus-strand synthesis reaches a maximum rate. We have isolated and characterized revertants of ts24, a member of the A complementation group of Sindbis HR mutants, that we had demonstrated previously to have a temperature-sensitive defect in the regulation of minus-strand synthesis. These revertants of ts24 replicated efficiently at 40° but nevertheless retained the temperature sensitive defect in the regulation of minus-strand synthesis: they continued to synthesize minus strands late in the replication cycle at 40° but not at 30° and in the presence or absence of protein synthesis. Although failure to regulate the synthesis of minus strands resulted in continuous minus-strand synthesis and in the accumulation of minus strands, the rate of plus-strand synthesis was not increased concertedly. Minus strands synthesized after the rate of plus-strand synthesis had become constant were demonstrated to be utilized as templates for 26 S mRNA synthesis. Thus, the change from an increasing to a constant rate of plus-strand synthesis during the alphavirus replication cycle cannot be governed solely by the number of minus strands that accumulate in infected cells. We present a model for the preferential utilization of minus strands as a mechanism for the cessation of minus-strand synthesis that occurs normally during alphavirus replication.