Transcription of the murine gammaherpesvirus 68 ORF73 from promoters in the viral terminal repeats.

Gammaherpesviruses persist as latent episomes in a dynamic lymphocyte pool. The regulated production of an episome maintenance protein is therefore crucial to their survival. The transcription initiation site of the murine gammaherpesvirus 68 episome maintenance protein, ORF73, was mapped to the viral terminal repeats, more than 10 kb distant from the open reading frame (ORF) itself. A 5' non-coding exon in the terminal repeats was spliced to the right end of the viral unique sequence, and then across ORFs 75a, 75b, 75c and 74 to ORF73. The right-hand portion of a single repeat unit was sufficient for constitutive promoter activity. The unique left end of the viral genome further enhanced ORF73 transcription. This, together with the large size of the predominant ORF73 mRNA, suggested that transcription initiates in distal repeat units and then splices between repeats to generate an extensive 5' untranslated region. A second promoter in the left-hand portion of the proximal terminal repeat unit generated a transcript which overlapped that of ORF73, but failed to splice to the ORF73 coding exon and so transcribed ORF75a. In distal repeat copies, however, transcription from this promoter would enter the next repeat unit to become an ORF73 mRNA. There was a third promoter just upstream of ORF73 itself. These data indicate that ORF73 transcription is highly complex, and support the idea that the terminal repeats of gamma-2-herpesviruses constitute a vital component of episomal persistence.