A CLUSTER OF LATENTLY EXPRESSED GENES IN KAPOSIS SARCOMA-ASSOCIATED HERPESVIRUS

Infection with Kaposi's sarcoma-associated herpesvirus (KSHV) is close ly associated with Kaposi's sarcoma (KS) and primary effusion lymphoma , with viral genomes present in a latent state in the majority of tumo r cells. Here we describe a cluster of latently expressed viral genes whose mRNAs are generated from a common promoter. Two mRNAs in this re gion encode the latency-associated nuclear antigen, the product of ope n reading frame 73 (ORF73). The larger RNA, of 5.8 kb, is an unspliced transcript that includes ORF72 and -71 at its 3' end; it initiates at nucleotides (nt) 127880 to 127886 from a promoter lacking recognizabl e TATA elements. A less abundant mRNA, of 5.4 kb, is a variant of this transcript, in which 336 nt of 5' noncoding information has been remo ved by RNA splicing. A third, more abundant RNA is generated from the same promoter region via splicing from the common splice donor at nt 1 27813 to an acceptor 5' to ORF72; this transcript is the presumed mRNA for ORF72, which encodes the viral cyclin D homolog. All three RNAs a re 3' coterminal. In situ hybridization analysis with probes that can detect all three transcripts shows that the RNAs are detectable in a l arge fraction of BCBL-1 cells prior to lytic induction and in > 70% of KS spindle cells in primary KS tumors. This confirms that these trans cripts are indeed latent RNAs and suggests a role for their products i n viral persistence and/or KSHV-associated proliferation.