Translation of open reading frame E5 of bovine papillomavirus is required for its transforming activity

A series of mutations in open reading frame (ORF) E5 of bovine papillomavirus type 1 has been constructed to determine whether this putative gene is required for in vitro oncogenic transformation by viral DNA. Frameshift mutations at either of two different positions located exclusively in ORF E5 cause a substantial reduction in the ability of the cloned viral DNA to induce the appearance of transformed foci of mouse C127 cells. A genetic mapping experiment with one of the mutants indicates that this characteristic transformation defect is actually due to the constructed mutation in ORF E5. Analysis of 10 different mutants with sequence changes at a single position in the ORF showed that there is an exact correspondence between transformation-competence and the ability of the 3' half of ORF E5 to be correctly translated. The transformation defect of an ORF E5 frameshift mutant can be suppressed by a second mutation that restores the correct reading frame to most of the ORF, but not by one that restores the reading frame near the 3' end of the ORF. These results constitute strong genetic evidence that translation of ORF E5 is required for efficient transformation of mouse C127 cells by bovine papillomavirus DNA. Wild-type ORF E5 has the potential to encode a short hydrophobic protein or polypeptide domain.