Further characterization of the latency-associated transcription unit of Marek's disease virus

Previous studies have identified a large (L) and a small (S) RNA transcript antisense to the MDV homologue of the ICP4 gene of herpes simplex virus (HSV) in cells infected with Marek's disease virus (MDV) and in lymphoblastoid cell lines. In this study the 5' and 3' ends of the L RNA and of the sense ICP4 transcript of MDV were mapped by Northern hybridization and RNase protection assays. The results showed that L RNA is approximately 10.6 kb and that the ICP4 sense transcript is initiated in the region of genomic DNA where the L RNA terminates whereas L RNA is initiated where the ICP4 transcript terminates. L RNA was abundant in chick embryo fibroblasts (CEF) infected with virus strain HPRS16/attenuated whereas S RNA was predominant in CEF infected with oncogenic HPRS16 and in RPL-1 cell line. Results of cycloheximide experiments showed that the ICP4 gene of MDV was transcribed as an immediate-early gene in infected CEF whereas transcription of the L RNA required protein synthesis. Sequencing of cDNA and Northern hybridization using oligonucleotide probes showed that S RNA shared similar intron/exon boundaries as the cDNAs from several cell lines indicating that there might be a relationship between the S RNA and the antisense transcripts that generated the cDNAs.