Recognition of cis-acting elements of infectious haematopoietic necrosis virus and viral hemorrhagic septicemia virus by homologous and heterologous helper proteins

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are grouped in the Novirhabdovirus genus within the family Rhabdoviridae. There are many similarities between these two viruses including the lengths of the leader and trailer regions and the homologies of the terminal sequences. We have developed two systems in which IHNV and VHSV minigenomes encoding the marker green fluorescent protein (GFP) can be expressed from plasmids by T7 RNA polymerase. These negative sense minigenome RNAs can be replicated, transcribed and packaged into infectious particles when coinfected with homologous helper viruses. After infection of the minigenome transfected BHK-T7 cells by heterologous helper viruses GFP expression was observed, but packaging of the minigenome RNAs into virus particles did not occur. Packaging of chimeric minigenomes by IHNV and VHSV was also not observed. Cotransfections of the negative sense minigenome plasmids with plasmids encoding nucleoprotein (N), phosphoprotein (P) and RNA polymerase (L) of IHNV and VHSV were carried out in all combinations. Minigenome constructs were expressed only after cotransfection with a set of helper plasmids (N, P and L) all originated from one virus. These results indicate that the cis-acting elements responsible for the encapsidation and transcription were recognized by the homologous and heterologous helper proteins, but packaging of the minigenome RNAs required homologous helper viruses.