Insertional inactivation of a fowlpox virus homologue of the vaccinia virus F12L gene inhibits the release of enveloped virions.

Insertion of the Escherichia coli lacZ gene into a ClaI restriction enzyme site of a 5.7 kb HindIII fragment of the fowlpox virus (FPV) genome resulted in the generation of stable recombinants. These recombinants produced plaques that were significantly smaller than those produced by parental FPV or by FPV recombinants containing the lacZ gene at other non-essential sites. Insertion of foreign DNA into the ClaI site disrupts a previously unidentified open reading frame (ORF) which potentially encodes a 74K polypeptide. The predicted amino acid sequence of this FPV ORF has 24% identity with the F12L ORF of vaccinia virus, the function of which is not currently known. Production of intracellular FPV was similar in cells infected with recombinant or parental viruses, but the number of infectious extracellular virions released into the medium by the recombinant was about 20% of that released by the parental virus. Likewise, the release of FPV particles, which were labelled in vivo with [3H]thymidine, was significantly lower in recombinant FPV-infected cells. These results suggest that the FPV homologue of the vaccinia virus F12L ORF is involved in the envelopment or release of infectious extracellular virions.