HPV-16 E7 functions at the G1 to S phase transition in the cell cycle

Previous reports have demonstrated that E7 is the major transforming gene of HPV-16 and that continued expression of the gene is required to maintain the transformed phenotype of primary baby rat kidney cells transformed by HPV-16 E7 and EJ-ras. To investigate the point of action of E7 in the cell cycle we have utilised a system of inducible expression of the E7 gene. The studies reported here show that stimulation of cellular DNA synthesis by E7 is distinct from that observed with calf serum. In combination with cytofluorimetric analyses these results indicate that E7 functions at the transition from G1 to S phase of the cell cycle. This adds further support to the hypothesis of a common pathway of transformation shared by the DNA tumour viruses HPV, SV40 and Adenovirus.