Transcription mapping and expression patterns of genes in the major immediate-early region of Kaposi's sarcoma-associated herpesvirus.

Viral immediate-early (IE) genes are the first class of viral genes expressed during primary infection or reactivation from latency. They usually encode regulatory proteins that play crucial roles in viral life cycle. In a previous study, four regions in the KSHV genome were found to be actively transcribed in the immediate-early stage of viral reactivation in primary effusion lymphoma cells. Three immediate-early transcripts were characterized in these regions, as follows: mRNAs for ORF50 (KIE-1), ORF-45 (KIE-2), and ORF K4.2 (KIE-3) (F. X. Zhu, T. Cusano, and Y. Yuan, 1999, J. Virol. 73, 5556-5567). In the present study, we further analyzed the expression of genes in these IE regions in BC-1 and BCBL-1 cells. One of the immediate-early regions (KIE-1) that encompasses ORF50 and other genes was intensively studied to establish a detailed transcription map and expression patterns of genes in this region. This study led to identification of several novel IE transcripts in this region. They include a 2.6-kb mRNA which encodes ORF48/ORF29b, a family of transcripts that are complementary to ORF50 mRNA and a novel K8 IE mRNA of 1.5 kb. Together with the IE mRNA for ORF50 which was identified previously, four immediate-early genes have been mapped to KIE-1 region. Therefore, we would designate KIE-1 the major immediate-early region of KSHV. In addition, we showed that transcription of K8 gene is controlled by two promoters, yielding two transcripts, an immediate-early mRNA of 1.5 kb and a delayed-early mRNA of 1.3 kb.