A mechanism for negative gene regulation in Autographa californica multinucleocapsid nuclear polyhedrosis virus

Abstract

The Autographa californica multinucleocapsid nuclear polyhedrosis virus (AcMNPV) ie-1 gene product (IE-1) is thought to play a central role in stimulating early viral transcription. IE-1 has been demonstrated to activate several early viral gene promoters and to negatively regulate the promoters of two other AcMNPV regulatory genes, ie-0 and ie-2. Our results indicate that IE-1 negatively regulates the expression of certain genes by binding directly, or as part of a complex, to promoter regions containing a specific IE-1-binding motif (5'-ACBYGTAA-3') near their mRNA start sites. The IE-1 binding motif was also found within the palindromic sequences of AcMNPV homologous repeat (hr) regions that have been shown to bind IE-1. The role of this IE-1 binding motif in the regulation of the ie-2 and pe-38 promoters was examined by introducing mutations in these promoters in which the central 6 bp were replaced with BglII sites. GUS reporter constructs containing ie-2 and pe-38 promoter fragments with and without these specific mutations were cotransfected into Sf9 cells with various amounts of an ie-1-containing plasmid (pIe-1). Comparisons of GUS expression produced by the mutant and wild-type constructs demonstrated that the IE-1 binding motif mediated a significant decrease in expression from the ie-2 and pe-38 promoters in response to increasing pIe-1 concentrations. Electrophoretic mobility shift assays with pIe-1-transfected cell extracts and supershift assays with IE-1-specific antiserum demonstrated that IE-1 binds to promoter fragments containing the IE-1 binding motif but does not bind to promoter fragments lacking this motif.