Key Concepts Of Section 113

Activators and Repressors of Transcription

■ Transcription factors, which stimulate or repress transcription, bind to promoter-proximal regulatory elements and enhancers in eukaryotic DNA.

■ Transcription activators and repressors are generally modular proteins containing a single DNA-binding domain and one or a few activation domains (for activators) or repression domains (for repressors). The different domains frequently are linked through flexible polypeptide regions (see Figure 11-18).

■ Among the most common structural motifs found in the DNA-binding domains of eukaryotic transcription factors are the C2H2 zinc finger, homeodomain, basic helix-loop-helix (bHLH), and basic zipper (leucine zipper). All these and many other DNA-binding motifs contain one or more a helices that interact with major grooves in their cognate site in DNA.

■ The transcription-control regions of most genes contain binding sites for multiple transcription factors. Transcription of such genes varies depending on the particular repertoire of transcription factors that are expressed and activated in a particular cell at a particular time.

■ Combinatorial complexity in transcription control results from alternative combinations of monomers that form heterodimeric transcription factors (see Figure 11-23) and from cooperative binding of transcription factors to composite control sites (see Figure 11-24).

■ Activation and repression domains in transcription factors exhibit a variety of amino acid sequences and three-dimensional structures. In general, these functional do

mains interact with co-activators or co-repressors, which are critical to the ability of transcription factors to modulate gene expression.

■ Cooperative binding of multiple activators to nearby sites in an enhancer forms a multiprotein complex called an enhancesome (see Figure 11-26). Assembly of en-hancesomes often requires small proteins that bind to the DNA minor groove and bend the DNA sharply, allowing bound proteins on either side of the bend to interact more readily.

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