Inducible and constitutive enhancer domains in the noncoding region of human papillomavirus type 18

The noncoding region of human papillomavirus type 18 (HPV-18) is shown to contain at least three enhancer elements. Two of these elements are responsive to papillomavirus-encoded trans-acting factors, and the third element functions as a constitutive enhancer, requiring only cellular factors for activity. The first enhancer (IE2) is located proximal to the E6 cap site and is responsive to papillomavirus E2 trans-activator. The second enhancer (IE6) is located approximately 500 base pairs upstream of the E6 cap site and is dependent upon the viral E6 gene product for function. A third enhancer (C) is located between 200 and 400 base pairs upstream of the E6 cap site and possesses a constitutive activity, requiring no HPV-18-encoded factors for function. The constitutive enhancer element exhibits some cell type preference for epithelial cell lines, but also functions in rodent fibroblast lines. Each of these enhancers manifests activity independent of the other elements and may reflect separate transcriptional control elements for different stages of the HPV-18 virus life cycle.