Two distinct cellular phosphoproteins bind to the c-fos serum response element

Induction of c-fos transcription by serum growth factors requires a 20 bp DNA sequence termed the serum response element (SRE). The SRE is a binding site for a nuclear protein, serum response factor (SRF). There is as yet no direct evidence that links SRF activity to SRE function. We show here that cells contain a second specific SRE-binding protein. It is distinguished from SRF by its size, chromatographic properties, binding specificity and contacts with nucleotides in the SRE. The presence in cells of two distinct proteins that can bind independently to the SRE suggests that the multiple functions of the SRE may be mediated by distinct proteins.