Non-essential loci in the BamHI-I and -F fragments of the HVT FC126 genome.

The sequence of BamHI-I fragment of the herpesvirus of turkeys (HVT) FC126 strain DNA was analyzed for the presence of potential open reading frames (ORFs). Four complete (ORFs 2 to 5) and 2 partial ORFs (ORFs 1 and 6) were detected. ORFs 2 and 3 were homologous to the HSV-1 UL55 and the EHV-1 gene 3, respectively. The ORF 6 was already partially sequenced by Smith et al. (Virology 207, 205-216, 1995), and was homologous to a Marek's disease virus (MDV) ORF located in a similar position (ORF 21; Ross et al., Virus Genes 7, 33-51, 1993a). No significant homology was found for the other ORFs. ORF 4 was antisense to ORF 3. Two HVT recombinants having an expression cassette inserted into two intergenic sites were generated and tested for viremia in chickens. Results demonstrated that these 2 intergenic loci are non-essential for in vitro and in vivo HVT replication. A 650 bp deletion in the repeat region flanking UL (TRL and IRL (BamHI-F)) has been identified in some DNA molecules of HVT FC126 strain. This deletion covers the entire truncated pp38 homologous ORF and the N-terminus of a small ORF which has no detectable homology with any known gene. Our results indicate that (1) this genomic region including the HVT pp38 homologue was not essential for in vitro and in vivo growth of HVT, and (2) this deletion had no apparent effect on Marek's disease (MD) protection induced by HVT.