Expression of the 39k promoter of Autographa californica nuclear polyhedrosis virus is increased by the apoptotic suppressor P35.

The 39k promoter of the baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) is transactivated by the viral protein IE1 and further stimulated by viral enhancer elements and the viral coactivator IE2. To identify additional viral proteins that regulate expression from the 39k promoter, we performed cotransfection experiments with clones of viral DNA and a plasmid containing the chloramphenicol acetyltransferase (CAT) gene under the control of the 39k promoter. Our results indicated that cotransfection of a 39cat construct with plasmids containing IE1 and the EcoRI-S fragment of viral DNA stimulated CAT activity 2.5-fold as compared to cells transfected in the absence of EcoRI-S. EcoRI-S encodes P35, a protein that suppresses apoptosis in AcNPV-infected cells. Primer extension analysis showed that the increase in CAT activity was due to a corresponding increase in mRNA, indicating that P35 may increase transcription from the 39k promoter. To determine whether P35 could activate the 39k promoter in the absence of IE1, the p35 open reading frame was cloned under the control of the ie1 promoter. Cotransfections of 39cat and IE-P35 in the presence and absence of pIE1 indicated that activation of 39k by P35 required the IE1 protein. Cotransfection of plasmids encoding P35 and IE1 did not lead to an increase in the expression of IE1. This suggests that the mechanism of P35 is different than that of IE2 which activates 39k indirectly by increasing the expression of IE1. Cotransfection experiments indicated that IE2 and P35 act in a synergistic fashion.