The 2.2 kb E1b mRNA of human Ad12 and Ad5 codes for two tumor antigens starting at different AUG triplets.

By nucleotide sequence analysis and S1 nuclease mapping we have determined the structural organization of early region E1b of Ad12. We have also revised the nucleotide sequence of the E1b region of Ad5. Both regions have an identical structural organization and show considerable homology at the nucleotide level. The major tumor antigens (Ad12, 19 and 54 kilodaltons [kd]); Ad5, 21 and 55 kd) are encoded in two overlapping reading frames. A single mRNA of 2.2 kilobases codes for both these proteins, depending on which AUG triplet serves as the start codon: the 19-21 kd protein initiates at the 5'-promximal AUG; the 54-55 kd protein initiates at the second AUG in another reading frame. Peptide mapping shows that the small and large tumor antigens do not share common tryptic peptides, in accordance with the nucleic acid sequence data. In addition, the 19-21 kd protein can also by synthesized from a one kilobase mRNA. Finally, the gene for the Ad12 analog of protein IX is characterized.