Protein IE1 is the product of a baculovirus gene, ie1, that is activated immediately upon entrance of the viral genome into the cell nucleus. This protein was previously shown to be a trans-regulator of viral genes whose products are required for initiation of the infectious cycle including viral DNA replication. To test whether the IE1 protein is also capable of trans-regulating nuclear genes of the host in vitro and in vivo, we transfected the ie1 gene of Bombyx mori nuclear polyhedrosis virus (BmNPV) into silkworm Bm5 tissue culture cells together with expression cassettes directing expression of chloramphenicol acetyl transferase or juvenile hormone esterase under the control of the cytoplasmic actin A3 gene promoter of B. mori. Cotransfection with the ie1 gene resulted in a dramatic increase in the amount of the two enzymes expressed in the transfected cells. The increased enzyme activities correlate with an increased accumulation of the corresponding mRNAs, and the latter is caused by an increase in the rate of transcription directed by the cytoplasmic actin gene promoter. The chromosomal cytoplasmic actin gene of Bm5 cells is also upregulated upon transfection of the cells with the ie1 gene. However, infection of cells with BmNPV does not cause an increase in the level of expression of the endogenous cytoplasmic actin gene. Thus, the effect of IE1 on the transcriptional properties of the cytoplasmic actin gene vary depending on whether IE1 is expressed in isolation or in the context of a viral infection. The trans-activating effects of BmNPV ie1 gene expression on the silkmoth actin promoter are also evident in Spodoptera frugiperda Sf21 and Choristoneura fumiferana Cf1 tissue culture cells. Finally, the ie1 gene of Autographa californica nuclear polyhedrosis virus can substitute for its BmNPV counterpart in all cell lines tested.