A synthetic early promoter from a baculovirus: roles of the TATA box and conserved start site CAGT sequence in basal levels of transcription.
Many baculovirus early genes and insect genes transcribed by RNA polymerase II have a conserved transcription start site sequence (CAGT) located downstream of a consensus TATA box. To examine the functions and interactions of these two motifs in initiating accurately positioned basal transcription, a 43-nt synthetic promoter was synthesized from the TATA box and start site sequences of the gp64 early promoter from the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV). The synthetic promoter initiated accurately and was also transactivated by the baculovirus transcriptional activator, IE1. To determine the roles of sequences within the 43-nt synthetic promoter, a series of linker-scanning and spacing mutations were analyzed for transcriptional activity, start site selection, and transactivation. Linker-scanning mutations were examined in vivo by transient expression and reporter gene assays. To examine transcription start site selection, promoter constructs were used for in vitro transcription in nuclear extracts from uninfected Spodoptera frugiperda (Sf9) cells. In vivo and in vitro analyses show that the TATA box, and not the start site CAGT, is the primary element controlling start site selection. Substitution of the conserved start site CAGT sequence resulted in a reduction of both reporter gene activity and in vitro transcripts, although transcripts initiated accurately. Data from linker-scanning and spacing mutations indicate that the conserved start site CAGT sequences are not required for accurate initiation but sequences at the start site play an important role in initiation efficiency.