Clusters of nuclear factor I binding sites identify enhancers of several papillomaviruses but alone are not sufficient for enhancer function

Abstract

The long control region (LCR) of human papillomaviruses (HPV) encompasses 5-12% of the viral genome and contains an intricate network of cis responsive elements. In the LCR of seven unrelated HPV-types, namely HPV-1, 6, 8, 11, 16, 18 and 33, we have identified clusters of 4 to 7 5-TTGGC-3 motifs suggesting nuclear factor I (NFI) binding sites. We randomly selected 20 (out of a total of 38) of these motifs and showed that pure NFI from porcine liver protects virtually the same nucleotides as a factor present in crude HeLa nuclear extracts. The footprints obtained with HeLa extracts in the LCR of HPV-16 are eliminated in competition experiments by an oligonucleotide representing the palindromic adenovirus NFI binding site. Restriction fragments from the genome of HPV-11, 16 and 18, which contain this cluster of NFI binding sites associated with binding sites of unrelated transcription factors, function as transcriptional enhancers. In contrast, a fragment from HPV-8 exhibiting exclusively NFI binding sites, or polymerized NFI sites from HPV-16, are functionally inactive. NFI seems to be necessary but not sufficient for HPV enhancer activation.