THE E2 TRANSCRIPTIONAL REPRESSOR CAN COMPENSATE FOR SP1 ACTIVATION OF THE HUMAN PAPILLOMAVIRUS TYPE-18 EARLY PROMOTER

The E6/E7 early promoter (P-105) of genital human papillomavirus type 18 contains binding sites for the viral regulator E2, tandemly repeated and closely flanked by two crucial promoter elements; the TATA box downstream and an Sp1 binding site upstream. We showed that binding of purified E2 and Sp1 proteins in vitro to their neighboring sites is mutually exclusive and that Sp1 is displaced by E2. However, this displacement did not result in repression of P-105 transcription. In contrast, binding of E2 to its site overlapping the Sp1 binding site activated transcription of P-105 derivatives lacking the E2 site most proximal to the TATA box. Surprisingly, a truncated form of E2, deleted of part of the transactivation domain and known as the E2 transcriptional repressor, as well as the E2 DNA-binding domain alone also supported transcription of these P-105 derivatives. In the context of P-105, the viral E2 protein can thus activate P-105 transcription in place of Sp1, even in the absence of its transactivation domain.