Adenovirus transforming 19-kD T antigen has an enhancer-dependent trans-activation function and relieves enhancer repression mediated by viral and cellular genes.

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Abstract
The adenovirus E1b region codes for two major tumor antigens of 19 kD and 55 kD, which are important for cell transformation. Our results indicate that the 19-kD T antigen possesses two enhancer-regulatory functions. It can trans-activate enhancer-linked promoters and relieve enhancer repression mediated by viral and cellular repressors. The 19-kD activation function enhances expression from different promoters linked to SV40, Py, Ela, and immunoglobulin heavy-chain enhancers. Enhancer activation by the 19-kD protein appears to be cell type-specific, since the heavy-chain and SV40 enhancers were not trans-activated in myeloma cells whereas the same enhancers were trans-activated in fibroblasts. The 19-kD enhancer activation function appears to be dominant over the enhancer repression function of E1a, since in cells expressing the 19-kD protein there is no significant repression despite a large increase in E1a expression. The 19-kD T antigen activates the Py enhancer in undifferentiated F9 cells indicating that the activation function of E1b masks enhancer repression by an "E1a-like" cellular gene product. The enhancer activation function of the 19-kD T antigen may be important for cell transformation and cell differentiation.