FIGURE 9-32 Combinatorial possibilities due to formation of heterodimeric transcription factors. (a) In some heterodimeric transcription factors, the activation domain of each monomer recognizes the same DNA sequence. In the hypothetical example shown, transcription-factor monomers A, B, and C can all interact with one another, creating six different alternative combinations of activation domains that can all bind at the same site. Each composite binding site is divided into two half-sites, and each heterodimeric factor contains the activation domains of its two constituent monomers. (b) When transcription-factor monomers recognize different DNA sequences, six alternative combinations of the transcription-factor monomers A, B, and C, each with a unique pair of activation domains, can bind to six different DNA sequences (sites 1–6). (c) Expression of an inhibitory factor (red) that interacts only with the dimerization domain of A inhibits binding; hence transcriptional activation at sites 1, 4, and 5 is inhibited, but activation at sites 2, 3, and 6 is unaffected.