DNA-binding activity of the adenovirus-induced E4F transcription factor is regulated by phosphorylation.

Previous experiments have identified E4F, an inducible cellular factor that binds to sequences in the adenovirus E4 promoter that are critical for E1A-dependent transcriptional activation. The E4F factor has been purified and shown to stimulate transcription in vitro from the E4 promoter. Analysis of the affinity-purified factor identifies a single polypeptide of 50 kD that has E4F-specific binding activity. E4F binding activity is also regulated during F9 cell differentiation and can be activated in differentiated F9 cells by viral infection. Furthermore, the activation process appears to involve a phosphorylation event, because treatment of E4F with alkaline phosphatase abolishes activity and incubation of the phosphatase-inactivated factor with an extract from virus infected cells restores activity.