Autoradiogram of Southern blot showing replicated (DpnI-resistant) viral DNAs from a transient replication assay with HPV 31 wild type (WT) and E6 mutants in SCC13 cells. Viral DNAs were excised from bacterial plasmid sequences, unimolecularly ligated, and transfected alone (lanes 1–6) or with equimolar amounts of HPV 31 E1 and E2 expression vectors (lanes 7–12). Low-molecular-mass DNA was isolated and analyzed by Southern blotting as described in Materials and Methods. DNA standards (Stds) are shown on the left and contain 500, 25, 2.5, and 0.5 pg of linearized wild-type HPV 31 DNA.