Ternary complex formation over the c-fos serum response element: p62TCF exhibits dual component specificity with contacts to DNA and an extended structure in the DNA-binding domain of p67SRF.

The serum response element (SRE) plays an essential role in the transcriptional regulation of proto-oncogene c-fos. A ternary complex, consisting of transcription factors p67SRF and p62TCF bound to the SRE is present in several cell types and its formation has been correlated with inducibility of the gene in different cells by serum, epidermal growth factor and phorbol esters. Interaction of p62TCF with the SRE in vitro exhibits both a degree of sequence specificity and a strict dependence on the presence of bound p67SRF. A 90 amino acid DNA-binding domain of p67SRF (coreSRF) suffices for ternary complex formation. DNase I footprinting and UV-mediated DNA-protein crosslinking experiments presented here show that direct DNA contacts are made by p62TCF with the 5' sequence of the SRE and thus explain the sequence dependence of ternary complex formation. Additionally, analysis of ternary complex formation by chimaeras of coreSRF and the related yeast protein ArgRI as well as comprehensive mutagenesis of non-conserved residues between the two proteins has yielded a coreSRF mutant specifically unable to interact with p62TCF and demonstrates that an extended structure in coreSRF is required for this interaction. Thus p62TCF exhibits dual component specificity in ternary complex formation over the c-fos SRE.