Alternative transcriptional initiation as a novel mechanism for regulating expression of a baculovirus trans activator.

In this report, we show that the Orgyia pseudotsugata nuclear polyhedrosis virus p34 gene, which is homologous to the Autographa californica nuclear polyhedrosis virus PE-38 gene, is a trans activator. The predicted p34 protein contains a number of motifs that are similar to those found in other eukaryotic transcriptional trans activators, including a putative zinc finger DNA-binding domain, a glutamine-rich domain, and a leucine zipper. Northern (RNA) blot analysis showed that the p34 gene is expressed as a 1.1-kb mRNA from 1 to 48 h postinfection and as a 0.7-kb mRNA from 18 to 120 h postinfection. Mapping of these transcripts showed that they were 3' coterminal but initiated at different 5' start sites. The 1.1-kb transcript initiates at a baculovirus early gene motif (CACAGT) and encodes the entire p34 open reading frame (ORF). The 0.7-kb transcript initiates at a baculovirus late gene start site (GTAAG) internal to the p34 ORF. Western blot (immunoblot) analysis using p34 antisera showed that the 0.7-kb transcript is translated as an amino-terminally truncated 20-kDa form of the full length 34-kDa protein. Functional analysis indicated that the 34-kDa protein transcriptionally trans activates the IE-2 promoter whereas the 20-kDa protein does not. Therefore, p34 produces two functionally different proteins from the same ORF, using the novel mechanism of alternative transcriptional initiation.