| Abstract: | Experiments exploring the reasons for a multiplicity of products from early region 1A of adenovirus 5 are described. Labeled early region 1A products from wild-type virus were synthesized in infected cells and in a cell-free system programmed with mRNA from infected cells, immunoprecipitated specifically with an antipeptide serum, E1A-C1, directed against the C-terminal sequence of E1A products, and separated by gel electrophoresis. Two-dimensional maps of [35S]methionine-labeled peptides were consistent with antigens of 52,000 daltons (52K) and 48.5K being from the 13S mRNA and antigens of 50K, 45K, and 35K from the 12S mRNA. Partial N-terminal sequences of 52K, 50K, 48.5K, and 45K synthesized in vitro showed that each of these antigens was initiated at the predicted ATG at nucleotide 560 in the DNA sequence. These results eliminate multiple initiation sites and proteolytic cleavage at the N-terminal end as sources of antigen diversity. Peptide maps and N-terminal sequences were obtained in a similar way for E1A products from the Ad5 deletion mutant dl1504, which lacks the normal initiator codon. As predicted, these polypeptides are initiated at the next ATG, 15 codons downstream in the wild-type sequence. These results are discussed in relation to Kozak's ribosomal scanning model. |
