Latent transcripts of Marek's disease virus are clustered in the short and long repeat regions

Marek's disease herpesvirus (MDV) induces tumors in chickens, and lymphoblastoid cells derived from such tumors contain the viral genome in a latent state and do not produce infectious virus. Poly(A)+ RNAs extracted from MDV-induced kidney lymphoma cells and from MKT-1 cells, a nonproducing lymphoblastoid cell line derived from a tumor induced by MDV, were electrophoretically separated under denaturing conditions, transferred to a solid substrate, and hybridized with labeled DNA probes representing approximately 95% of the virus genome. These analyses revealed 29 viral RNA transcripts in the kidney lymphoma and 32 viral RNAs in MKT-1 cells. In both instances, the transcripts hybridized to a restricted region comprising approximately 20% of the MDV genome located in the repeats flanking the long and short unique sequences and in the adjacent unique sequences. The sizes of the transcripts derived from kidney lymphoma and MKT-1 cells and the distributions of the homologous regions in the viral genome were very similar. The most abundant transcripts were homologous to the BamHI I2 fragment. The results suggest that expression of the MDV genome in MKT-1 cells closely reflects the expression of the MDV genome in tumors. The gene expression over extended regions of the genome is in concordance with that observed in latent gammaherpesviruses rather than that observed in latent alphaherpesviruses.