Novel regulatory properties of the IE1 and IE0 transactivators encoded by the baculovirus Autographa californica multicapsid nuclear polyhedrosis virus

The baculovirus Autographa californica multicapsid nuclear polyhedrosis virus expresses two immediate-early genes from the HindIII-G region (map units 90.4 to 96.8) of the genome. During the early phase of infection, nonspliced 1.9-kb and spliced 2.1-kb transcripts are expressed which encode the IE1 and IE0 (spliced IE1) gene products, respectively. These two gene products differ only in that IE0 contains an additional 54 amino acids at the amino terminus. RNA analysis of these two genes during infection revealed that they were differentially expressed. IE1 was expressed early and late, whereas IE0 was expressed only early in infection. The regulation of these two immediate-early genes was analyzed by transient expression assays. The IE1 gene product stimulated expression of IE1 promoter-directed expression but down-regulated expression from the IE0 promoter. The IE0 gene product also transactivated the IE1 promoter but did not affect expression from its own promoter. Unlike IE1, which transactivates the delayed early 39K gene in the presence and absence of the homologous region (hr) enhancers, IE0 transactivated the 39K promoter only in the presence of cis-linked hr5 enhancer. The results of this study in conjunction with previous results suggest that the IE1 gene encodes a multifunctional gene product that may be involved in (i) repression of immediate-early gene expression, (ii) continued expression of its own gene product during infection, and (iii) transactivation of the delayed early and late classes of genes.