The cellular transcription factor E2f requires viral E1A and E4 gene products for increased DNA-binding activity and functions to stimulate adenovirus E2A gene expression.

Whereas a wide variety of cellular proteins interact with the cis-regulatory elements of the adenovirus E1A and E2A genes, only the DNA-binding activity of the cellular E2f factor is modulated by viral early-gene expression. An analysis of cellular E2f protein levels and adenovirus early-gene expression in a panel of independently cloned virus-transformed rodent cell lines and in virus-infected rodent cells has established that both the E1A 289-amino-acid (289R) protein and a yet-to-be-defined E4 gene product are required for maximal E2f DNA-binding activity. To distinguish between the multiple roles the E1A protein could serve in this process, the E2f DNA-binding activity was determined in a virus-transformed cell line which contains a conditional-lethal mutation affecting the 289R protein. Since E4 gene expression was not altered by the incubation conditions, the observation of reduced cellular E2f activity at the nonpermissive temperature suggests a direct role for the E1A 289R protein in E2f activation. When a virus containing a deletion in the E4 gene was introduced into cell lines which can complement the E4 gene defect, a correlation between high cellular E2f levels and increased rates of E2A gene transcription was observed. A time course analysis of the viral infection revealed that E2f functions catalytically to stimulate viral E2A gene transcription. These observations have led to several hypotheses concerning possible mechanisms by which elevated E2A gene expression, which leads to cytotoxicity, might be avoided in the transformed cell.