Identification and characterization of human herpesvirus 6A strain GS cDNAs homologous to the human cytomegalovirus UL36-38 gene family
Human herpesvirus 6 is a clinically relevant herpesvirus that is the causative agent of the childhood disease exanthem subitum (roseola). HHV-6 like all herpesviruses becomes latent after the acute infection, and the possibility of reactivation to the lytic phase of growth remains for the lifetime of the host. Reactivated HHV-6 infections are very often found in individuals suffering from acquired immunodeficiency syndrome (AIDS) caused by the human immunodeficiency virus (HIV). HHV-6 preferentially infects a subset of T lymphocytes, CD4+ T cells, the same population infected by HIV, and the potential exists for direct interaction between these two viruses. HHV-6 has been shown to activate expression of a reporter gene regulated by the primary HIV promoter located within the long terminal repeat (LTR). This activation was in part mediated by a single open reading frame (ORF) called B701. HHV-6 B701 is the 3′ half of the ORF now identified as U16. HHV-6 and HCMV are both classified as [special characters omitted] herpesviruses and one of the characteristics linking these two viruses is the colinear arrangement of the genes in the HHV-6 unique region and the genes in the HCMV unique long region. U16 bears restricted homology to an HCMV essential gene, UL36, which is expressed as a singly spliced transcript. It was predicted that U16 would be expressed as part of a larger spliced transcript. B701 was used to screen an HHV-6A (strain GS) cDNA library and two different but overlapping transcripts were identified. One of these contained a single ORF formed by splicing together U16 with the ORF immediately upstream, U17. The transcript encodes a protein of 38kDa and antibodies raised against the B701 protein recognized the in vitro generated 38kDa protein and several proteins from HHV-6A (GS) infected cells. The other cDNA contained U16 and U15, but they were not spliced together. The antibodies against B701 also recognized proteins generated in vitro from this cDNA. Several species of RNA were recognized by each cDNA on Northern blots, suggesting that like HCMV UL36, these transcripts are part of a family of transcripts. 5′ extension analysis identified two potential transcriptional start sites upstream of the first methionine found in the U17/U16 ORF. The 5′ extension studies identified an additional cDNA that demonstrated a unique splicing event, which allowed for the expression of the complete U16 ORF without U17. The products of the HCMV gene family that includes UL36 (UL36–38) are differentially regulated. RT-PCR studies demonstrated that the U17/U16 gene family is also differentially regulated with some transcripts classified as immediate early transcripts, while others are late transcripts.