Baculovirus gp64 gene expression: analysis of sequences modulating early transcription and transactivation by IE1

Expression of the baculovirus major envelope glycoprotein gene (gp64) is regulated by transcription from both early and late promoters. To characterize the early promoter and identify sequences involved in the regulation of gp64 early transcription, promoter-reporter gene fusions were generated from the Orygia pseudotsugata nuclear polyhedrosis virus gp64 promoter and were analyzed by transient expression in uninfected insect cells. For these analyses, 5' deletion mutations were constructed in the gp64 upstream regulatory region. Larger promoter constructs were functional in uninfected Lymantria dispar cells, indicating that transcription from the gp64 early promoter required no additional viral gene products. Deletion analysis of the gp64 upstream region revealed several regulatory regions. These included a putative negative regulatory element between -319 and -166 nucleotides (nt) and multiple positive regulatory elements between -166 and -77 nt. Deletion of the TATA box located between -77 and -62 nt resulted in the loss of transcriptional activity. Cotransfections of reporter constructs and a plasmid containing a baculovirus transcriptional transactivator gene (Autographa californica nuclear polyhedrosis virus IE1) resulted in transcriptional transactivation of all constructs containing an intact TATA box. These data demonstrate that sequences upstream of the gp64 TATA box are not essential for IE1 transactivation and that only 34 nt upstream of the early transcription start site were necessary for basal levels of transcription and for transactivation by IE1. Function of the gp64 early promoter was also examined in cell lines from Spodoptera frugiperda and Drosophila melanogaster.