An activating transcription factor (ATF)-binding site (consensus sequence 5'-GTGACGTACAG-3') is a promoter element present in a wide variety of viral and cellular genes. The two best-characterized classes of genes that contain ATF sites are E1A-inducible adenoviral genes and cAMP-inducible cellular genes. Here, we report the isolation of eight ATF cDNA clones, each of which is derived from a separate gene. All ATF cDNA clones examined contain a leucine zipper motif and are significantly similar to one another only within this region. The leucine zipper region of ATF proteins is also similar to that of the AP-1/c-jun family of transcription factors, whose DNA-binding site differs from the ATF-binding site at a single position. DNA binding studies reveal two mechanisms for generating further diversity from the ATF proteins. First, some, but not all, combinations of ATF proteins form heterodimers that efficiently bind to DNA. Second, although all ATF proteins bind to the ATF site, their precise interactions with DNA differ from one another, as evidenced by methylation interference analysis. Our results help to explain how a single promoter element, an ATF site, can be present in a wide variety of promoters.