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 spliced 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 capacities of partial cDNAs representing three of these mRNAs indicated their abilities to function in E2 protein translation, although at different efficiencies. In the present study, the transcription-regulatory activities of the E2 cDNAs towards the virus long control region (LCR) have been examined. LCR regulation was evaluated in transient transfection assays by using the chloramphenicol acetyltransferase reporter gene linked to the HPV-16 LCR. Transfections were carried out into fibroblast (Cf2Th) and epithelial (C33A) cell lines. It is shown that all three E2 cDNAs transrepressed the virus LCR in a dose-dependent manner. Transrepression was mainly dependent on the function of the E2 ORF and was abolished or markedly reduced by premature termination or truncation of the E2 ORF. Transrepression activities exhibited by the various E2 cDNAs correlated with the previously defined efficiencies of E2 protein translation from the respective templates. The truncated E2 cDNAs exhibited variable low regulatory activities that correlated with the activities of the 5' ORFs contained in each cDNA. The E6I and E1C ORFs transactivated the virus LCR whereas the E6IV cDNA transrepressed LCR activity. Thus the 5' ORFs contribute in different manners to the overall activities of the polycistronic cDNAs.