Octamer and heat shock elements regulate transcription from the AcMNPV polyhedrin gene promoter

The baculovirus expression vector system exploits the polyhedrin (polh) promoter for high expression of foreign proteins in insect cells. The mechanism of basal and hyperactivated transcription from this promoter, however, remains poorly understood. We have analyzed the 4-kb upstream region of the polh promoter; deletion of two separate parts of the 4-kb upstream region, harboring the Oct binding site and the heat shock element, respectively, resulted in significant reduction of reporter gene expression regulated by the polh promoter. Insect cell host factors could bind to these elements in vitro. Moreover, these elements could activate polh transcription during viral infection when present upstream of a minimal polh promoter in transient expression reporter assays. Our results suggest the possible existence of transcription factors belonging to the POU and heat shock transcription factor family in Spodoptera frugiperda cells and support the hypothesis that host proteins may play a major role in activating transcription from the polh promoter.