Genetic content of a 20.9 kb segment of human herpesvirus 6B strain Z29 spanning the homologs of human herpesvirus 6A genes U40-57 and containing the origin of DNA replication.

A continuous 20.9 kb sequence from human herpesvirus 6 variant B (HHV-6B) strain Z29 (GenBank accession number L16947) is genetically colinear with a discrete segment of the human cytomegalovirus (HCMV) UL region and with HHV-6 variant A (HHV-6A). Short nucleotide sequence determinations at multiple sites within an 8.5 kb region immediately 3' to the 20.9 kb contig revealed additional colinearity between HHV-6B, HCMV and HHV-6A. Homology studies with the predicted peptide sequences from 11 complete and 12 partial HHV-6B open reading frames (ORFs) revealed that most encode proteins conserved to varying degrees in all previously sequenced primate herpesviruses. HHV-6B homologs were identified for the HSV-1 ICP18.5, ICP8, UL52, UL24, UL25 and major capsid protein. Several HHV-6B proteins had limited amino acid similarity to their positional homologs in other herpesviruses. Each gene identified is highly homologous to its HHV-6A counterpart, including two unique HHV-6 genes predicted to encode membrane-associated glycoproteins. However, two regions of substantial divergence were noted, one spanning the origin of replication and the other encoding one of the putative HHV-6-specific glycoprotein genes. Substitutions in the latter region lead to predicted differences in reading frames and protein lengths among HHV-6 isolates.

Comparison of a 20 kb region of human herpesvirus 6B with other human beta herpesviruses reveals conserved replication genes and adjacent divergent open reading frames.

The sequence of a 20.15 kb region from human herpesvirus 6 variant B (HHV-6B) strain Z29 is described (GenBank accession number L14772). Determinations of protein homologies for seventeen predicted gene products revealed HHV-6B homologs of six proteins well-conserved both in genetic context and amino acid sequence throughout the alpha-, beta-, and gammaherpesvirus subfamilies. These include proteins involved in viral DNA replication, packaging and nucleotide metabolism, and conserved proteins of undefined function. The close evolutionary relationship of the human betaherpesviruses, HHV-6B, HHV-6A, HHV-7 and human cytomegalovirus (HCMV) was confirmed by identification of several protein sequences encoded only by these viruses, including homologs of the HCMV early phosphoprotein family and a series of HCMV open reading frames predicted to encode glycoprotein exons. Homologs of essential HSV-1 replication proteins, UL8 and UL9, were also identified. Downstream from the conserved replication locus, each betaherpesvirus contains a region of divergent, small open reading frames. The evolution of this region and its potential use in the development of a viral vector system are discussed.

A strongly immunoreactive virion protein of human herpesvirus 6 variant B strain Z29: identification and characterization of the gene and mapping of a variant-specific monoclonal antibody reactive epitope.

We previously identified a 101-kDa apparent molecular mass polypeptide (101K) as the major immunoreactive virion protein of human herpesvirus 6 variant B strain Z29 [HHV-6B(Z29)] and found that the human immune response to this protein is HHV-6-specific (Yamamoto, M., Black, J. B., Stewart, J. A., Lopez, C., and Pellett, P. E., 1990, J. Clin. Microbiol. 28, 1957-1962). We report here the identification and characterization of the gene encoding 101K. We found 81% amino acid identity between an HHV-6B(Z29) open reading frame (ORF) and its homolog in HHV-6A strain U1102 [HHV-6A(U1102)]. The product of this gene was identified as 101K on the basis of both the reactivity of a 101K-specific monoclonal antibody (MAb C3108-103) with a bacterially expressed portion of the gene and the reactivity of polyclonal rabbit antibodies raised against the bacterially expressed protein with 101K expressed by HHV-6B(Z29)-infected cells. MAb C3108-103 reacted with eight of eight variant B isolates and none of six variant A isolates, indicating that it is a variant-specific MAb. The MAb reactivity was mapped to an eight-amino-acid segment of 101K. HHV-6A(U1102) differs from HHV-6B(Z29) by two amino acids in this region; substitution mapping with synthetic oligopeptides mapped the variant B specificity to Asp723, this explaining the failure of the MAb to react with variant A proteins. A set of transcripts appropriately sized for expression of 101K was identified and precisely mapped. The transcripts originated down-stream from either of two TATA boxes located 139 bp apart in the region 5' to the 101K ORF, with one 5'-species being much more abundant. Two independent polyadenylation sites were identified; the canonical polyadenylation signal located 3' to the 101K ORF was used much more frequently than was the atypical polyadenylation signal located within the 101K ORF. These results suggest a complex regulatory mechanism for this gene.