Effects of baculovirus transactivators IE-1 and IE-2 on the Drosophila heat shock 70 promoter in two insect cell lines.

Baculovirus infection of permissive cells proceeds in a cascade fashion with the transcription of early, late and very late genes. The structure of a number of baculovirus early gene promoters has been dissected in detail and the viral factors necessary to stimulate their expression have been identified. Early baculovirus gene promoters in general have a resemblance to host promoters while late and very late gene promoters are different from early baculovirus promoters and are more defined. In this study we investigated whether two key Autographa californica M nucleopolyhedrovirus (AcMNPV) transactivators have the ability to regulate the commonly used cellular promoter from the Drosophila heat shock 70 protein gene, during transient gene expression assays in two insect cell lines permissive for AcMNPV infection, SF-21 and TN-368, or during viral infection. The AcMNPV ie-1 transactivator gene stimulated gene expression of this cellular promoter in both cell lines when the promoter was cis-linked to an enhancer element, but stimulation in the absence of enhancer elements was either undetected or lower than in the presence of enhancer elements in SF-21 and TN-368 cells, respectively. The transactivator ie-2 stimulated gene expression in the presence of cis-linked enhancer elements and ie-1 in SF-21 cells. During viral infection, the heat shock 70 promoter was maximally activated at 12 hours post infection. We discuss how these results affect the interpretation of transient gene expression assays performed in the presence of viral transcription factors.