The truncated C-terminal E2 (E2-TR) protein of bovine papillomavirus (BPV) type-1 is a transactivator that modulates transcription in vivo and in vitro in a manner distinct from the E2-TA and E8^E2 gene products.

The E2 open reading frame of bovine papillomavirus (BPV)-1 encodes a 410 amino acid (aa) transcriptional activator, E2-TA, and collinear polypeptides--E2-TR (243 aa) and E8^E2 (196 aa). E8^E2 and E2-TR share the DNA-binding domain of E2-TA, and both have been defined as transcriptional repressors. Although purified E2-TR and E8^E2 proteins specifically bound E2 sites with similar affinities, only the E2-TR stimulated transcription. Here we show that E2-TR trans-activates E2-dependent promoters 5 to 10-fold in cooperation with cellular factors and in a dose-dependent fashion in epithelial cells and fibroblasts of animal or human origin while E2-TA activated >100-fold and the E8^E2 had no effect. However, in contrast to E2-TA, E2-TR activated transcription from a promoter-proximal position. E2-TR also partially inhibited the BPV-1 P89 or heterologous promoters whereas E8^E2 led to complete repression. Thus, the BPV-1 E2-TR modulates viral gene expression in a manner distinct from other E2 proteins.

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