Transcriptional regulation by Fos and Jun in vitro: interaction among multiple activator and regulatory domains

The proteins encoded by the proto-oncogenes c-fos and c-jun (Fos and Jun, respectively) form a heterodimeric complex that regulates transcription by interacting with the DNA-regulatory element known as the activator protein 1 (AP-1) binding site. Fos and Jun are members of a family of related transcription factors that dimerize via a leucine zipper structure and interact with DNA through a bipartite domain formed between regions of each protein that are rich in basic amino acids. Here we have defined other domains in the Fos-Jun heterodimer that contribute to transcriptional function in vitro. Although DNA-binding specificity is mediated by the leucine zipper and basic regions, Jun also contains a proline- and glutamine-rich region that functions as an ancillary DNA-binding domain but does not contribute directly to transcriptional activation. Transcriptional stimulation in vitro was associated with two regions in Fos and a single N-terminal activation domain in Jun. These activator regions were capable of operating independently; however, they appear to function cooperatively in the heterodimeric complex. The activity of these domains was modulated by inhibitory regions in Fos and Jun that repressed transcription in vitro. In the context of the heterodimer, the Jun activation domain was the major contributor to transcriptional stimulation and the inhibitory regions in Fos were the major contributors to transcriptional repression in vitro. Potentially, the inhibitory domains could serve a regulatory function in vivo. Thus, transcriptional regulation by the Fos-Jun heterodimer results from a complex integration of multiple activator and regulatory domains.