HOST-DEPENDENT EVOLUTION OF THE SINDBIS-VIRUS PROMOTER FOR SUBGENOMICMESSENGER-RNA SYNTHESIS

Alphaviruses are alternately transmitted between arthropod and vertebr ate hosts. In each host, the virus transcribes a subgenomic mRNA that encodes the viral structural proteins which encapsidate the genome to form progeny virions, Transcription initiates at an internal site call ed the promoter, To determine if promoter utilization varies in mammal ian versus mosquito cells, we used these cells as hosts to select for active promoters among a library of different mutant promoters, Compar ed with that in BHK-21 cells, selection was more rapid in mosquito (C7 -10) cells, with much less diversity of promoters remaining after fewe r passages, Thus, promoter selection is host dependent. With further p assaging, both BHK-21 and C7-10 cells selected for similar sequences t hat closely resemble the wild-type promoter sequence, The difference i n the rates of selection is not because BHK-21-derived promoters canno t function in mosquito cells. Instead, part of the host dependence is probably due to posttranscriptional differences between BHK-21 and C7- 10 cells that may require more active promoters in mosquito cells, Par t of the host dependence may also be attributed to the decreased rate of transcription versus that of replication in mosquito cells, This ch ange in regulation of subgenomic to genomic RNA synthesis appears to c orrelate with the extent of cleavage or pausing of the genomic RNA syn thesis at or close to the promoter.