INRA, Unité de Virologie et Immunologic Moléculaires, Jouy-en-Josas, France.
An mRNA differential display methodology was used to study the rainbow trout response to viral infection. A new transcript (vig-2) induced by viral haemorrhagic septicaemia virus (VHSV) in rainbow trout leucocytes was identified from the head-kidney. vig-2 was also induced in vivo during experimental infection and following DNA immunisation with a plasmid containing a gene encoding the viral glycoprotein. Viral induction of vig-2 was blocked by cycloheximide (CHX), indicating its dependency on a newly synthesised intermediate protein. This intermediate protein is most probably related to interferon because treatment of cells with a conditioned medium displaying an interferon-like activity resulted in a strong vig-2 expression, which was not blocked by CHX treatment. The cDNA sequence of the vig-2 transcript displays several mRNA destabilisation motifs and two signals characteristic of immediate-early gene expression. Curiously, vig-2 has no evident encoding potential except for a small 51 amino acid putative polypeptide with no clear similarity to any sequence available in the databanks. Therefore, the complete vig-2 genomic sequence was determined from a lambda phage clone retrieved from a genomic DNA library of rainbow trout. The genomic organisation of vig-2 shows five exons delimited with typical splice acceptor and donor sites. A promoter with a canonical ISRE, confirming that vig-2 is an interferon-responsive gene, is also present 115 nt upstream of the first exon.