The papillomavirus E2 protein contains an amino-terminal region thought necessary and sufficient to support transcriptional activation and a carboxy-terminal region shown to direct sequence-specific DNA binding and dimerization. A cysteine residue in the center of the E2 DNA recognition helix is highly conserved among papillomavirus E2 proteins. Mutations of this cysteine in bovine papillomavirus type 1 E2 to serine and glycine resulted in proteins which failed to activate E2-dependent promoters in mammalian cells. These E2 mutants were DNA-binding competent, dimeric, and nuclear. When fused to the VP16 transactivation domain, C-terminal regions of E2 containing the mutations at 340 supported transcriptional activation, indicating that the heterologous trans-activation domain did not require cysteine in the DNA-binding helix as did the full-length E2 transactivating protein. Although cysteine-340 was required for transcriptional activation it was not required for DNA replication in vivo. Together, these results suggest that the E2 DNA-binding domain may directly contribute to functions of transcriptional activation previously thought limited to the N-terminal domain.
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