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We have sequenced 1730 bp of human papilloma virus type 18 (HPV 18) DNA containing the open reading frames (ORF) E6, E7, the N‐terminal part of E1 and, additionally, 120 bp of the N‐terminal part of L1. Based on these sequencing data, together with the human papilloma virus type 16 (HPV 16) DNA sequence published recently, we identified and cloned the ORF E6, E7, E1 and L1 of HPV 18 and the ORF E6, E7, E1, E4, E5, L2 and L1 of HPV 16 into prokaryotic expression vectors. The expression system used provides fusions to the N‐terminal part of the MS2 polymerase gene controlled by the heat‐inducible lambda PL promoter. Using the purified fusion proteins as immunogens we raised antisera against the proteins encoded by the ORF E6, E7 and E1 of HPV 18 as well as those encoded by the ORF E6, E7, E4 and L1 of HPV 16. By Western blot analysis we could show that the E7 gene product is the most abundant protein in cell lines containing HPV 16 or HPV 18 DNA. It is a cytoplasmic protein of 15 kd in the SiHa and the CaSki cell lines which contain HPV 16 DNA, and 12 kd in the HeLa, the C4‐1 and the SW756 cell lines which contain HPV 18 DNA. These results were confirmed by in vitro translation of hybrid‐selected HPV 16 and HPV 18 specific poly(A)+ RNA from SiHa, CaSki and HeLa cells. Additionally, these experiments led to the identification of an 11‐kd E6 and a 10‐kd E4 protein in the CaSki cell line as well as a 70‐kd E1 protein in HeLa cells.

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Heteroduplex DNA molecules were prepared in vitro using one strand of DNA carrying a point mutation and one strand of the corresponding wild-type DNA. The heteroduplex DNA was transfected into competent bacteria and the progeny genotypes in the resulting infective centers were determined. From the results we conclude that about 80% of all transfected DNA molecules are repaired before DNA replication starts. This fraction of repaired DNA is independent of the location of the mismatched nucleotide pair. However, mismatch correction occurs preferentially on the H strand of the heteroduplex DNA. The repair does not depend on a known phage coded function but requires the active bacterial genes mutU, mutH, mutS and probably mutL.

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