Transcriptional silencer of the human papillomavirus type 8 late promoter interacts alternatively with the viral trans activator E2 or with a cellular factor

Abstract

The noncoding region of the highly oncogenic, epidermodysplasia verruciformis-associated human papillomavirus type 8 contains a negative regulatory element (NRE). Quantitative RNase protection analysis confirmed that the NRE sequence acts as a silencer of transcription. A 38-bp sequence upstream of late promoter P7535 down-regulated expression from the homologous P7535 promoter, as well as the heterologous tk gene promoter, independently of its orientation relative to the test promoters. It also reduced gene expression when cloned downstream of the transcription units. Transient expression assays with keratinocytes and fibroblasts of epidermodysplasia verruciformis patients and controls demonstrated that the NRE activity is not cell specific. Gel retardation tests suggested that NRE specifically interacts with only one nuclear factor. Mutational analysis identified three NRE mutants which no longer formed a detectable DNA-protein complex but still repressed transcription, indicating that protein-DNA interaction is not relevant for the silencer function. The NRE contains a binding site of viral trans activator protein E2. It was shown that expression of E2 overrides the inhibitory effect of the NRE sequences. Binding of E2 and that of the cellular factor were mutually exclusive. The bifunctional nature of NRE acting as a silencer and a target site for viral trans activator E2 offers an interesting opportunity to regulate the switch from early to late transcription in the human papillomavirus life cycle.