TRANSIENT EXPRESSION OF THE AUTOGRAPHA-CALIFORNICA NUCLEAR POLYHEDROSIS-VIRUS IMMEDIATE-EARLY GENE, IE-N, IS REGULATED BY 3 VIRAL ELEMENTS

Autographa californica nuclear polyhedrosis virus (AcMNPV) is a double-stranded DNA virus that expresses several immediate-early genes under the control of different promoters. The expression of one of these transcription units, IE-N, is shown here, by a transient expression assay, to be regulated by both cis- and trans-acting viral elements. The steady-state levels of IE-N mRNA were very abundant soon after infection but were nearly undetectable during the late phase of the viral life cycle. Analysis of the transient expression of a reporter construct driven by the IE-N promoter (IE-NCAT) was conducted to define viral elements which regulate IE-N gene expression. Viral enhancer hr1 and two immediate-early genes, IE-1 and IE-N, were shown to affect relative levels of reporter enzyme activity produced by IE-NCAT. The hr1 enhancer stimulated the expression of IE-NCAT, independent of orientation and position relative to the promoter and in the absence of any trans-acting viral factors. Regulation of IE-NCAT expression by the IE-1 and IE-N genes required less than 290 bp of promoter sequences upstream of the site of transcription initiation and was not dependent upon the hr1 enhancer. Coexpression of the IE-N gene had an autostimulatory effect upon IE-NCAT activity, whereas coexpression of the IE-1 gene reduced levels of reporter activity. The levels of reporter activity measured upon coexpression of either immediate-early gene with IE-NCAT linked to the hr1 enhancer appear to be the combined result of both cis- and trans-regulatory elements influencing expression from IE-NCAT. These results suggest that IE-N gene expression in baculovirus infection may be influenced by the concerted activity of three AcMNPV regulatory elements.