Bidirectional transcriptional promoters in the vaccinia virus genome

Vaccinia virus intermediate and late class transcriptional promoters each have two essential sequence elements: an initiator at the transcriptional start site and an upstream core element. Many of the transcription units in the viral genome are oriented divergently with insufficient nucleotides between the start of the open reading frames to accommodate two separate upstream core elements in their promoters. This raises the possibility that two promoters could share essential elements. Reporter gene experiments were used in this study to document examples of promoter arrangements in which two late promoters share a core element and another in which a late promoter shares a core element with an intermediate promoter. Another arrangement in which the core element of one late promoter is the initiator of the other is shown. Nucleotide replacements in the initiator element of a bidirectional promoter lead to activation of the other, suggesting that bidirectional promoter arrangement is a mechanism of attenuating promoter strength.