C ATF, A MEMBER OF THE ACTIVATING TRANSCRIPTION FACTOR FAMILY OF DNA-BINDING PROTEINS, DIMERIZES WITH CAAT ENHANCER-BINDING PROTEINS AND DIRECTS THEIR BINDING TO CAMP RESPONSE ELEMENTS/

Members of the C/EBP family of basic-leucine zipper (bZip) transcripti on factors form heterodimers and bind to the CAAT box and other sequen ce-related enhancer motifs. Using a P-32-labeled protein probe consist ing of the bZip domain of C/EBPbeta, we isolated a clone encoding C/EB P-related ATF (C/ATF), a bZip protein that heterodimerizes with C/EBP- like proteins but belongs to the CREB/ATF family. C/ATF homodimers do not bind to typical C/EBP DNA sites. Instead they bind to palindromic cAMP response elements such as that of the somatostatin gene. In addit ion, C/ATF-C/EBPbeta heterodimers bind to a subclass of asymmetric cAM P response elements exemplified by those in the phosphoenolpyruvate ca rboxykinase and proenkephalin genes. Transient transfection studies in dicate that interactions between C/ATF and C/EBPbeta are the basis for a functional cross talk between these two families of transcription f actors that may be important for the integration of hormonal and devel opmental stimuli that determine the expression of subsets of genes in specific cellular phenotypes.