Transcriptional activator proteins have multiple, separable domains. (a) The Gal4 protein has two domains and forms a dimer. (b) The experimental removal of the activation domain shows that DNA binding is not sufficient for gene activation. (c) Similarly, the bacterial LexA protein cannot activate transcription on its own, but, when fused to the Gal4 activation domain (d), it can activate transcription through LexA-