FIGURE 10.02 Transcription Factors Have Two Independent Domains

A) One domain of a GAL4 transcription factor normally binds to the GAL4 DNA recognition sequence and another binds to the transcription apparatus. B) If the LexA sequence is substituted for the GAL4 site, the transcription factor does not recognize the DNA and no binding occurs. C) An artificial protein made by combining a LexA binding domain with a GAL4 activator domain will not recognize the GAL4 binding site, but D) will bind to the LexA recognition sequence and activate transcription. Thus, the GAL4 activator domain acts independently of any particular recognition sequence. It works as long as it is held in close contact to the DNA.

The mediator consists of a constant core that is similar in yeast and higher eukary-otes. Attached to this are other subunits that vary between organisms and also between different tissues within the same organism. Many individual mediator proteins were identified as "co-activator" proteins before it was realized that they belong together in a complex.

Enhancer sequences loop around to contact the transcription apparatus.

Insulator sequences prevent enhancers from interfering with the wrong genes.

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