Characterization of a putative Spodoptera exigua multicapsid nucleopolyhedrovirus helicase gene

Putative baculovirus helicases have been implicated as playing an important role in viral DNA replication and host specificity. The Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) helicase is therefore of interest since the virus only infects the beet army worm. Sequence analysis of the SeMNPV lef5-p39 (mu 46.5-55.1) region, which is collinear with the 39K-lef5 area in Autographa californica MNPV (AcMNPV), revealed an open reading frame (ORF) of 3666 bp potentially encoding a protein with a molecular mass of 143 kDa. This protein had considerable amino acid sequence similarity (58%) to AcMNPV p143, including seven conserved motifs characteristic of helicases. In cultured insect cells, this SeMNPV ORF is expressed from 4 to 12 h postinfection and its major transcript of 4 kb starts 11 to 12 nt upstream of the putative translational initiation site (ATG). To study their possible role in the specificity of baculovirus DNA replication, the putative AcMNPV and SeMNPV helicase genes were tested for their ability to replicate homologous regions (hrs; putative origins of DNA replication) in a transient DNA replication assay in insect cells. All viral cis- and trans-acting factors were provided as plasmids using either Achr2 or Sehr1 as the DNA replication origin. SeMNPV p143 could not substitute for AcMNPV p143 in the transient assays supplemented with either hr. Similar results were obtained when the SeMNPV and AcMNPV ie1 genes were exchanged. None of the essential AcMNPV trans-acting factors could be complemented by SeMNPV infections to support DNA replication of hrs. These data suggest a specific interaction between baculovirus DNA replication factors to form the replisome and/or between the replisome and the origin of DNA replication.