Interplay of viral and cellular proteins along the long control region of human papillomavirus type 18.

The long control region of human genital papillomavirus type 18 harbors transcription regulatory elements, such as the E6 promoter and a cell type-specific enhancer independent of the E2 protein. By performing DNase I footprint experiments in vitro with protein extracts from different cell lines and tissues we searched for cellular factors interacting with the totality of the control region. We detected a total of eight different protected sites; most of them were found with all the extracts. Two of these sites, one in the enhancer and the other in the sequences proximal to the E6 cap site, interact with a member of the activator protein 1 family. However, in the absence of fine mutational analysis, we cannot readily discern an exact functional role for the different binding sites. We also characterized two regions, which are protected only in the presence of E2, corresponding to the perfect palindromes ACCGN4CGGT and present either 500 nucleotides or tandemly repeated about 70 nucleotides upstream of the E6 cap site. This last protected area covers a large part of the putative TATA box of the E6 promoter and could explain its repression by bovine papilloma virus type 1 E2, which could interfere with binding of the TFII D ubiquitous transcription factor to the TATA sequence of the promoter.