Transcription initiation in eukaryotes is controlled by nucleoprotein complexes formed through cooperative interactions among multiple transcription regulatory proteins. These complexes may be assembled via stochastic collisions or defined pathways. We investigated the dynamics of Fos-Jun-NFAT1 complexes by using a multicolor fluorescence resonance energy transfer assay. Fos-Jun heterodimers can bind to AP-1 sites in two opposite orientations, only one of which is populated in mature Fos-Jun-NFAT1 complexes. We studied the reversal of Fos-Jun binding orientation in response to NFAT1 by measuring the efficiencies of energy transfer from donor fluorophores linked to opposite ends of an oligonucleotide to an acceptor fluorophore linked to one subunit of the heterodimer. The reorientation of Fos-Jun by NFAT1 was not inhibited by competitor oligonucleotides or heterodimers. The rate of Fos-Jun reorientation was faster than the rate of heterodimer dissociation at some binding sites. The facilitated reorientation of Fos-Jun heterodimers therefore can enhance the efficiency of Fos-Jun-NFAT1 complex formation. We also examined the influence of the preferred orientation of Fos-Jun binding on the stability and transcriptional activity of Fos-Jun-NFAT1 complexes. Complexes formed at sites where Fos-Jun favored the same binding orientation in the presence and absence of NFAT1 exhibited an 8-fold slower dissociation rate than complexes formed at sites where Fos-Jun favored the opposite binding orientation. Fos-Jun-NFAT1 complexes also exhibited greater transcription activation at promoter elements that favored the same orientation of Fos-Jun binding in the presence and absence of NFAT1. Thus, the orientation of heterodimer binding can influence both the dynamics and promoter selectivity of multiprotein transcription regulatory complexes.M any transcription regulatory proteins form heterodimers that bind to palindromic DNA sequences. Such heterodimers can potentially bind their recognition sites in either of two opposite orientations (1-5). Heterodimers that bind in opposite orientations present different protein surfaces for interactions with adjacent DNA binding proteins. Thus, the orientation of heterodimer binding may influence interactions with other transcription factors. Opposite orientations of heterodimer binding can result in different transcriptional activities (6-9). The mechanisms whereby the orientation of heterodimer binding controls transcriptional activity have not been defined.Fos and Jun family basic region-leucine zipper (bZIP) proteins regulate the expression of different genes in different cell types through cooperative interactions with other transcription factor families (10). Complexes formed by Fos and Jun with members of the NFAT family at composite regulatory elements induce cytokine gene expression in T cells activated by antigen presentation (11,12). The x-ray crystal structures of DNA-bound complexes formed by the bZIP domains of Fos and Jun alone as well as by the DNA binding domains of Fos, Jun...