Characterization of the Helicoverpa armigera and Pseudaletia unipuncta granulovirus enhancin genes.

Enhancins are baculovirus proteins capable of enhancing infections in insect larvae by other baculoviruses. We have identified the enhancin proteins in four species of granulovirus (GV). In this paper we describe the cloning and sequencing of the enhancin genes of the Pseudaletia unipuncta granulovirus-Hawaiian strain (PsunGV-H) and the Helicoverpa (Heliothis) armigera granulovirus (HearGV). The PsunGV-H enhancin gene is virtually identical to the previously characterized Trichoplusia ni GV (TnGV) enhancin gene. In contrast, a comparison of the predicted amino acid sequences of TnGV enhancin (901 amino acids) and HearGV enhancin (902 amino acids) revealed an overall identity of only 80%, with greater conservation (88%) from amino acids 1-550. Primer extension analysis of enhancin RNAs identified the baculovirus late promoter motif that serves as the transcriptional start site in the HearGV enhancin gene. It is located three nucleotides from the putative enhancin translational initiator codon. RNase protection analysis demonstrated that both read-through and termination occur at the 3' end of the gene. Since a partial open reading frame (ORF) was identified immediately downstream of the 3' end of the enhancin ORF, these data suggested that a sizeable fraction of the enhancin mRNAs may be bi-cistronic and share a common 3' end with a downstream transcription unit.