We have shown previously that a chemically synthesized adenovirus E1A region 3 peptide of 49 amino acids, protein domain 3 (PD3; residues 140 to 188 of the 289-amino-acid protein), trans activates viral genes in vitro and in vivo. To study structure-function relationships, we synthesized N-terminal deletion and cysteine substitution mutant peptides and tested their activities in a cell microinjection assay. Peptides lacking 1 to 12 N-terminal residues exhibited 5- to 50-fold-reduced molar specific activities, whereas those lacking 16 or 18 residues were inactive. Substitution of each of five PD3 cysteine residues with alanine resulted in substantial losses of activity: mutants in the PD3 N-terminal portion showed 40 to 55% of wild-type activity but required a 20-fold-higher concentration than PD3, whereas those in the C-terminal half were as much less active. These peptide mutant studies suggest the existence of two PD3 functional regions: one, localized in the C-terminal 70 to 75% of the molecule, is essential for trans activation; the other, localized in the N-terminal 25 to 30%, can be overridden to a significant extent at high peptide concentrations
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