Spatial flexibility in ternary complexes between SRF and its accessory proteins.

We investigated the sequence requirements for ternary complex formation by the transcription factor SRF and its Ets domain accessory factors Elk-1 and SAP-1. Ternary complex formation is specified by an SRF consensus site CC(A/T)6GG and a neighbouring Ets motif (C/A)(C/A)GGA(A/T), which is contacted by Elk-1/SAP-1. Both the spacing of these sequences and their relative orientation can be substantially altered with little effect on the efficiency of ternary complex formation. Efficient ternary complex formation by Elk-1 is mediated by the B box, a conserved 21 amino acid region located 50 residues C-terminal to the Ets domain, which also acts to inhibit autonomous DNA binding. Binding studies with the isolated Ets domains indicate that ternary complex formation compensates for low affinity Ets domain-DNA interactions. Several naturally occurring SREs containing Ets motifs at different locations to that in the human c-fos SRE allow SAP-1 and Elk-1 recruitment in vitro. We discuss the mechanism of ternary complex formation.