Overexpression of the E1B 55-kilodalton (482R) protein of human adenovirus type 12 appears to permit efficient transformation of primary baby rat kidney cells in the absence of the E1B 19-kilodalton protein.

To analyze the structure and function of the E1B 19,000-molecular-weight protein (19K protein) (163R) of human adenovirus type 12, mutants were produced at various positions across the 163R-coding sequence. Viruses bearing mutations within the first 100 or so amino acids yielded unstable 163R-related products, induced DNA degradation and enhanced cytopathic effect (cyt/deg phenotype) in KB cells, and transformed primary rodent cells at much lower efficiencies than wild-type (wt) virus. Deletion of the final 16 residues at the carboxy terminus had no phenotypic effect. Alteration of residue 105 reduced transforming efficiency significantly, suggesting that this region of 163R is functionally important. Disruption of the AUG initiation codon at nucleotide 1542 blocked production of 163R completely but resulted in higher levels of E1B 55K-482R protein synthesis and a transforming efficiency similar to that of wt virus. These data suggested that while 163R is of some importance, normal transforming efficiencies can be obtained in its absence if 482R is overexpressed.