FIGURE 3-13 A molecular machine: the transcription initiation complex. The core RNA polymerase, general transcription factors, a mediator complex containing about 20 subunits, and other protein complexes not depicted here assemble at a promoter in DNA. The polymerase carries out transcription of DNA; the associated proteins are required for initial binding of the polymerase to a specific promoter. The multiple components function together as a molecular machine.