Characterization of host range factor 1 (hrf-1) expression in Lymantria dispar M nucleopolyhedrovirus- and recombinant Autographa californica M nucleopolyhedrovirus-infected IPLB-Ld652Y cells.

We previously identified a gene, host range factor 1 (hrf-1), in Lymantria dispar M nucleopolyhedrovirus (LdMNPV) which promoted Autographa californica M nucleopolyhedrovirus (AcMNPV) replication in a nonpermissive cell line IPLB-Ld652Y (Ld652Y). A recombinant AcMNPV, vAcLdPS, that bore hrf-1 controlled by two synthetic baculovirus late promoters and that replicated in Ld652Y cells was constructed. In this study, we constructed a new recombinant AcMNPV, vAcLdPD, bearing only hrf-1 controlled by its own promoter. vAcLdPD replicated in Ld652Y cells in the same manner as vAcLdPS, confirming that hrf-1 alone was sufficient to promote AcMNPV replication in Ld652Y cells. hrf-1 was transcribed as a delayed early gene in LdMNPV but as an immediate early gene in both recombinant AcMNPVs. Primer extension analysis showed that the initiator sequence TCAGT was used as the transcription start site in both LdMNPV and recombinant AcMNPVs. Additional sequencing revealed several regulatory motifs in the hrf-1 upstream region. hrf-1 transcripts in LdMNPV- and vAcLdPS-infected Ld652Y cells terminated near the polyadenylation signal at the end of hrf-1 ORF while in vAcLdPD, the hrf-1 transcripts terminated at a downstream polyadenylation signal at the end of ORF 603. Using Western blot analysis, we detected HRF-1 expression in both recombinant AcMNPV-infected Ld652Y cells but not in LdMNPV-infected Ld652Y cells.