Comparison of the complete DNA sequences of human herpesvirus 6 variants Aand B

Human herpesvirus 6 (HHV-6), which belongs to the betaherpesvirus subfamily and infects mainly T cells in vitro, causes acute and latent infections. T wo variants of HHV-6 have been distinguished on the basis of differences in several properties. We have determined the complete DNA sequence of HHV-6 variant B (HHV-6B) strain HST, the causative agent of exanthem subitum, and compared the sequence with that of variant A strain U1102. A total of 115 potential open reading frames (ORFs) were identified within the 161,573-bp contiguous sequence of the entire HHV-6 genome, including some genes with r emarkable differences in amino acid identity. All genes with <70% identity between the two variants were found to contain deleted regions when ORFs th at could not be expressed were excluded from the comparison. Except in the case of U47, these differences were found in immediate-early/regulatory gen es, DR2, DR7, U86/90, U89/90, and U95, which may represent characteristic d ifferences of variants A and B. Also, we have successfully typed 14 differe nt strains belonging to variant A or B by PCR using variant-specific primer s; the results suggest that the remarkable differences observed were conser ved evolutionarily as variant-specific divergence.