The divergence between two oncogenic Herpesvirus saimiri strains in a genomic region related to the transforming phenotype.

Herpesvirus saimiri strains can be divided into at least three subgroups (A, B, C) based on sequence divergence at the left end of viral unique sequence DNA. Strains of subgroups A and C are highly oncogenic and readily transform simian T-lymphocytes in vitro to interleukin-2 independent growth, while subgroup B strains do not. A left terminal reading frame of a H. saimiri subgroup A strain was shown previously to correlate with the oncogenic phenotype and in vitro transforming potential; the deduced polypeptide was termed STP-A. Furthermore, this same region contains an open reading frame (ORF) for dihydrofolate reductase (DHFR) and genes for five virus-specific U RNAs (HSURs). We now show by sequence analysis of the corresponding region in a subgroup C strain that DHFR and HSUR genes are present in both virus subgroups; however, no sequence homologous to the STP-A reading frame was found in this subgroup C virus. At a position and orientation similar to STP-A, two ORFs were found for peptides sharing a putative transmembrane domain. One of them encodes a peptide with collagen-like repetitions. In addition to the lack of similarity to STP-A, these two reading frames also did not show any similarity to known oncogenes. The organization of sequences at the left junction of unique L- and repetitive H-DNA of H. saimiri suggests frequent recombinational events, possibly accelerating the uptake of foreign genes by the virus.