Early region 1A (E1A) of adenoviruses (Ad) codes for potent activator and repressor molecules which are involved in the regulation of viral and cellular gene expression. Gene regulatory functions of E1A proteins are mainly located in their conserved regions (CR) 1 to 3. In addition to the CRs, specific amino acids (aa) of the N-terminal end play an important role in some gene regulatory functions. We describe here the identification and characterization of a novel trans-activation domain which is located in the non-conserved N-terminal end of Ad12 E1A, namely aa 1-29. Fusion of this region to the DNA-binding domain of the yeast transcription factor Gal4 generates a strong trans-activator which induces gene expression of reporter constructs in dependence on Gal4 DNA-binding sites. Furthermore, transient expression assays using the physiological E1A-responsive adenoviral E2 early promoter revealed that the N terminus is involved in its activation. The gene regulatory function of the N terminus is specific for E1A proteins of the highly oncogenic serotype Ad12, as the respective E1A N terminus of the non-oncogenic serotype Ad2 is unable to activate the expression of the reporter gene as Gal4 fusion protein. Moreover, deletion mutant analyses demonstrate that Ad12 E1A proteins carry three independently acting activation domains: (1) aa 1-29, (2) CR1 and (3) CR3.