An element downstream of the transcription start site is required for activation of Bombyx mori nucleopolyhedrovirus bro-c promoter

Summary.

 Bombyx mori nucleopolyhedrovirus (BmNPV) contains five baculovirus repeated ORF (bro) genes. We have previously reported that all of these genes (bro-a, b, c, d and e) are transcribed as early genes and require viral factor(s) for their expression. In this study, we investigated the mechanism of promoter activation of the bro-c gene. Transient expression assays using genomic libraries of BmNPV indicated that the baculoviral trans-regulator IE-1 is responsible for activating the bro-c promoter. To identify essential site(s) for promoter activation, mutations were introduced to the promoter region of bro-c. Interestingly, it was shown that the pentanucleotide sequence CACGC located 30 nucleotides downstream of the RNA start site was essential for bro-c promoter activation. In addition, the RNA start site and the spacing between the RNA start site and CACGC were also required for promoter activation. By introducing a CACGC sequence into the corresponding region of the bro-b promoter, which is not normally trans-activated by IE-1, we demonstrated that this pentanucleotide motif has the ability to confer trans-activation by IE-1 on a promoter. Gel retardation experiments also showed a sequence-specific DNA binding protein induced by baculovirus infection interacts with the CACGC motif.