Temporal Mapping of Transcripts in Herpesvirus 6 Variants

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Abstract
To define the molecular features characteristic of the early stages of infection of lymphocytes with human herpesvirus 6 (HHV-6) variant A or B, we studied the temporal regulation of expression of selected sets of viral genes. Thus, U42, U94, U89-U90, U73, and U39 are α genes since their transcripts (i) were made in the presence of inhibitors of protein synthesis and (ii) were detected 3 h after infection of untreated cells. U41, U53, U31, and U19 are β genes since their expression is inhibited by cycloheximide but not by phosphonoacetate, an inhibitor of DNA synthesis. U100 is a γ gene since its spliced transcript encoding the structural glycoprotein gp82/105 was first detected 16 h after infection of untreated cells but could not be detected in cells treated with phosphonoacetate. HHV-6 variants differ in the transcription patterns of their genes. U16-U17 originates a splice transcript and is regulated as α in HHV-6B and as β in HHV-6A. U91 generates two transcripts, amplified as 476- and 374-bp PCR fragments. The 476-bp fragment is α in HHV-6A-infected cells but β in HHV-6B-infected cells. Conversely, the 374-bp fragment is β in HHV-6A-infected cells and α in HHV-6B-infected cells. Furthermore, the spliced product of U18-U19-U20 (526 bp) is β in HHV-6A-infected cells, but only a partially spliced form (1.9 kb) was detected at late stages of infection in HHV-6B. HHV-6 transcription was also studied in nonproductive lymphoid cells, and the same transcription pattern detected during lytic infection was observed. Also, HHV-6 variants maintain the differences in U91, U16-17, and U18-U19-U20. We conclude that, as expected from the sequencing data, gene expression is generally similar in HHV-6 variants. However, transcription of selected genes in HHV-6A and HHV-6B differs with respect to temporal regulation and splicing pattern. Furthermore, the identification of viral functions expressed during the different stages of lytic replication suggests that reverse transcription-PCR for HHV-6 genes is a useful diagnostic approach to differentiate between latent and productive HHV-6 infection.