Transcription-modulatory activities of differentially spliced cDNAs encoding the E2 protein of human papillomavirus type 16

Human papillomavirus (HPV) type 16 expresses a variety of alternatively spl iced polycistronic mRNAs encoding the E2 transcription-regulatory protein. These mRNAs initiate at the p97 promoter and contain the 880/2708 (a-type), 880/2581 (a'-type) and 226/2708 (d-type) splice sites upstream from the E2 open reading frame (ORF). Recent studies investigating the translational c apacities of partial cDNAs representing three of these mRNAs indicated thei r abilities to function in E2 protein translation, although at different ef ficiencies. In the present study, the transcription-regulatory activities o f the E2 cDNAs towards the virus long control region (LCR) have been examin ed. LCR regulation was evaluated in transient transfection assays by using the chloramphenicol acetyltransferase reporter gene linked to the HPV-16 LC R. Transfections were carried out into fibroblast (Cf2Th) and epithelial (C 33A) cell lines. It is shown that all three E2 cDNAs transrepressed the vir us LCR in a dose-dependent manner. Transrepression was mainly dependent on the function of the E2 ORF and was abolished or markedly reduced by prematu re termination or truncation of the E2 ORF. Transrepression activities exhi bited by the various E2 cDNAs correlated with the previously defined effici encies of E2 protein translation from the respective templates. The truncat ed E2 cDNAs exhibited variable low regulatory activities that correlated wi th the activities of the 5' ORFs contained in each cDNA. The E6I and E1C OR Fs transactivated the virus LCR whereas the E6IV cDNA transrepressed LCR ac tivity. Thus, the 5' ORFs contribute in different manners to the overall ac tivities of the polycistronic cDNAs.