Cyclic AMP (cAMP) is an intracellular second messenger that activates transcription of many cellular genes. A palindromic consensus DNA sequence, TGACGTCA, functions as a cAMP-responsive transcriptional enhancer (CRE). The CRE binds a cellular protein of 38 kD in placental JEG-3 cells. A placental lambda gt11 library was screened for expression of specific CRE-binding proteins with the CRE sequence as a radioactive probe. A cDNA encoding a protein of 326 amino acids with the binding properties of a specific CRE-binding protein (CREB) was isolated. The protein contains a COOH-terminal basic region adjacent to a sequence similar to the "leucine zipper" sequence believed to be involved in DNA binding and in protein-protein contacts in several other DNA-associated transcriptional proteins including the products of the c-myc, c-fos, and c-jun oncogenes and GCN4. The CREB protein also contains an NH2-terminal acidic region proposed to be a potential transcriptional activation domain. The putative DNA-binding domain of CREB is structurally similar to the corresponding domains in the phorbol ester-responsive c-jun protein and the yeast transcription factor GCN4.