Hairpin formation within the enhancer region of the human enkephalin gene.

The 3',5'-cyclic adenosine monophosphate (cAMP)-inducible enhancer of the human enkephalin gene is located within an imperfect palindrome of 23 base pairs. We have found that a 23-base-pair oligonucleotide duplex containing the enhancer undergoes a reversible conformational transition from the duplex to two individual hairpin structures each formed from one strand of the duplex. Each individual hairpin forms with mismatched base pairs, one containing two GT pairs and the other containing two AC pairs. The conformational transition is stabilized by proton transfer to the hairpin containing AC mismatched pairs. The unique physical and thermodynamic properties of the enkephalin enhancer DNA suggest a model in which DNA secondary structure within the enhancer region plays an active role in cAMP-inducible activation of the human enkephalin gene via formation of cruciform structures.