A single site mutation in a truncated Fos protein allows it to interact with the TRE in vitro.

The Fos and Jun proteins, which are components of the transcription factor AP1, associate through the interaction of their so-called leucine zipper domains and bind strongly and specifically to DNA at phorbol ester-responsive elements. Jun also homodimerizes and binds the same element whereas Fos seems to have no specific affinity for DNA. We show that a single amino-acid change in the leucine zipper of Fos is sufficient to allow a truncated Fos protein to homodimerize and thus form a complex with DNA, even in the absence of Jun. This Fos-derived homodimer recognizes the consensus phorbol-ester responsive element specifically, in vitro. We conclude that the structural requirements for specific DNA binding are present in the Fos protein itself, with the exception of its lack of self-affinity.