CHARACTERIZATION OF A PUTATIVE SPODOPTERA-EXIGUA MULTICAPSID NUCLEOPOLYHEDROVIRUS HELICASE GENE

Putative baculovirus helicases have been implicated as playing an impo rtant role in viral DNA replication and host specificity, The Spodopte ra exigua multicapsid nucleopolyhedrovirus (SeMNPV) helicase is theref ore of interest since the virus only infects the beet army worm, Seque nce analysis of the SeMNPV lef5-p39 (mu 46.5-55.1) region, which is co llinear 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 consider able amino acid sequence similarity (58%) to AcMNPV p143, including se ven conserved motifs characteristic of helicases, In cultured insect c ells, this SeMNPV ORF is expressed from 4 to 12 h postinfection and it s 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 an d SeMNPV helicase genes were tested for their ability to replicate hom ologous regions (hrs; putative origins of DNA replication) in a transi ent DNA replication assay in insect cells, All viral cis-and trans-act ing factors were provided as plasmids using either Achr2 or Sehr1 as t he DNA replication origin. SeMNPV p143 could not substitute for AcMNPV p143 in the transient assays supplemented with either hr, Similar res ults were obtained when the SeMNPV and AcMNPV ie 1 genes were exchange d, None of the essential AcMNPV trans-acting factors could be compleme nted by SeMNPV infections to support DNA replication of hn. These data suggest a specific interaction between baculovirus DNA replication fa ctors to form the replisome and/or between the replisome and the origi n of DNA replication.