DNA structure determines protein binding and transcriptional efficiency of the proenkephalin cAMP-responsive enhancer.

Two precisely arranged proenkephalin cAMP response elements (CREs) behave as a single protein binding site. The experiments described support a model in which a secondary structural change creates a new binding site, which is made up of sequences from both of the CREs. The CRE-binding protein (CREB) binds CRE-1, but binding there is entirely dependent on the presence of CRE-2. Electron spectroscopic images show that a CREB dimer occupies twice as much DNA in the proenkephalin gene as in the prodynorphin gene. The enhancer region is sensitive to P1 nuclease in a CREB concentration-dependent manner, and sensitivity is strand-specific, indicating protein-stabilized structural change. DNase I analysis shows that in the native proenkephalin gene, CREB binds both CRE-1 and CRE-2. In vivo, both CREs are occupied in the transcriptionally active proenkephalin gene, while neither is in the silent gene. Whereas CREB can bind CRE-2, mutation or elimination of either proenkephalin CRE alters response to second messengers and transcription factors. Thus, binding to CRE-2 alone is not sufficient. Specific and efficient transcription of the proenkephalin gene requires the presence of both CREs, precisely arranged to allow them to form a single protein binding site.