Virus-specific transcription in a Herpesvirus saimiri-transformed lymphoid tumor cell line

Herpesvirus saimiri-transformed lymphoid tumor cell lines contain nonintegrated covalently closed circular viral DNA molecules in high multiplicity. One of those cell lines, 1670, carries large viral DNA circles (202 kilobase pairs) with two stretches of repetitive DNA (70.8% G + C) that are interspersed between two segments of unique DNA (36% G + C). Since it was not known if there is any viral gene expression in H. saimiri-transformed cells, we initiated a study of transcription in cell line 1670. cDNA was generated by reverse transcription of cellular RNA and hybridized with cloned virion DNA fragments. The experiments indicated that appreciable transcription is restricted to a single segment of unique DNA. This sequence is present once only in the circular viral DNA and corresponds to unique DNA between map units 0.89 and 0.93 of virion DNA. By Northern blot hybridizations with labeled cloned probes of virion unique DNA, one predominant virus-specific polyadenylated transcript of, at most, 2.7 kilo-bases could be detected in tumor cell line 1670. The direction of transcription was determined by hybridization with randomly primed cDNA and, in parallel, with oligodeoxythymidylate-primed cDNA probes. Apparently, the patterns of virus-specific RNA synthesis in the H. saimiri-transformed cells are clearly distinct from the transcription program in other herpesvirus transformation systems analyzed before.