Effects of HPV16 E1 mutants on HPV18 replication. (A) Schematic representation of HPV16 E1 mutants. F, FLAG-tag; ND, N-terminal domain; DBD, DNA-binding domain; OD, oligomerization domain; HD, helicase domain. (B) C33A cells were transfected with 1 ng of the HPV18 gDNA together with the indicated amounts (ng) of the expression plasmids. Three days after transfection, low molecular weight DNA was isolated by the Hirt procedure and digested with Dpn I. The Dpn I-resistant HPV18 gDNA was quantified by real-time PCR and normalized to the luciferase gene. The level of the replication was presented as the relative amount of the Dpn I-resistant DNA compared to that obtained by the replication with the HPV18 E1/E2 alone. Each bar represents the average of two independent experiments with the standard error of mean.