Serum induction of human c-fos gene transcription is mediated by a complex of proteins bound to the upstream serum response element (SRE). Genomic footprinting studies from our lab have shown that a region larger than the dyad-symmetrical portion of the SRE is protected from methylation (Herrera, Shaw and Nordheim, 1989, Nature 340, 68). The protection is unchanged prior to, during, and post induction, indicating that possibly modification of a preexisting complex is the mechanism of induction. Binding studies suggest that this assembly contains a dimer of the serum response factor (SRF), the ternary complex p62^(TCF) (Shaw, Schro¨ter and Nordheim, 1989, Cell 56, 563) and a factor bound to the AP1-like element immediately downstream of the SRE. The SRF has been implicated in mediating both repression of transcription from the fos promoter (Shaw, Frasch and Nordheim, 1989, EMBO J. 8, 2567 and activation by serum. Efficient serum induction also involves another factor, p62^(TCF), that interacts with a SRF dimer and the SRE in vitro and stabilizes the complex, perhaps synergistically (Schro¨ter, Mueller, Meese and Nordheim, 1990, EMBO J. 9, 1123). A fragment of SRF comprising the region from amino acids 132 to 222 ("core") can also form such a ternary complex. SRF stimulates transcription in SRF-depleted nuclear extracts, as well as in untreated extracts. Both purified SRF and recombinant vaccinia-generated SRF are functional, whereas recombinant "core" has only a weak effect. We are presently studying the roles of SRF, p62^(TCF) and AP1 in this in vitro stimulation, as well as determining the domains of SRF involved in activation, repression and ternary complex formation with p62^(TCF). Our aim is to reconstitute serum induction of the fos promoter in vitro.