YY1, a multifunctional protein essential for embryonic development, is a known repressor or activator of transcription. In cardiac and skeletal myocytes, YY1 has been described essentially as a negative regulator of muscle-specific genes. In this study, we report that YY1 is a transcriptional activator of the B-type natriuretic peptide (BNP) gene, which encodes one of the heart major secretory products. YY1 binds an element within the proximal cardiac BNP promoter, in close proximity to the high affinity binding sites for the zinc finger GATA proteins. We show that YY1 cooperates with GATA-4 to synergistically activate BNP transcription. Structure-function analysis revealed that the DNA binding domain of YY1 is sufficient for cooperative interaction with GATA-4, likely through corecruitment of the CREB-binding protein coactivator. The results suggest that YY1 and GATA factors are components of transcriptionally active complexes present in cardiac and other GATA-containing cells.