SUMMARY

• 37.1 Eukaryotic Cells Have Three Types of RNA Polymerases

  There are three types of RNA polymerase in the nucleus: RNA polymerase I makes ribosomal RNA precursors, II makes messenger RNA precursors, and III makes transfer RNA precursors. Eukaryotic promoters are complex, being composed of several different elements.

• 37.2 RNA Polymerase II Requires Complex Regulation

  Promoters for RNA polymerase II can be located on the 5′ side or the 3′side of the start site for transcription. One common type of eukaryotic promoter consists of a TATA box centered between −30 and −100 and paired with an initiator element. Eukaryotic promoter elements are recognized by proteins called transcription factors rather than by RNA polymerase II. The saddle-shaped TATA-box-binding protein unwinds and sharply bends DNA at TATA-box sequences and serves as a focal point for the assembly of transcription complexes. The TATA-box-binding protein initiates the assembly of the active transcription complex. The activity of many promoters is greatly increased by enhancer sequences that have no promoter activity of their own. Enhancers are DNA elements that can modulate gene expression from more than 1000 bp away from the start site of transcription. Enhancers are often specific for certain cell types, depending on which DNA-binding proteins are present.

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  Most eukaryotic genes are not expressed unless they are activated by the binding of specific transcription factors to sites on the DNA. These specific DNA-binding proteins interact directly or indirectly with RNA polymerases or their associated proteins.

• 37.3 Gene Expression Is Regulated by Hormones

  The estradiol receptor is a member of the nuclear hormone-receptor family, a group of receptors that regulate gene expression in response to hormone binding. These receptors are modular: they consist of separate DNA-binding and hormone-binding domains. The receptor recognizes particular DNA sequences, called response elements, upstream of the genes that they regulate. On hormone binding, the hormone receptor recruits coactivators that alter the activity of RNA polymerase II.

• 37.4 Histone Acetylation Results in Chromatin Remodeling

  Eukaryotic DNA is tightly bound to basic proteins called histones; the combination is called chromatin. Changes in chromatin structure play a major role in regulating gene expression. Among the most important functions of coactivators is to catalyze the addition of acetyl groups to lysine residues in the tails of histones. Histone acetylation decreases the affinity of the histones for DNA, making additional genes accessible for transcription. In addition, acetylated histones are targets for proteins containing specific binding units called bromodomains. Bromodomains are components of two classes of large complexes: (1) chromatin-remodeling engines and (2) factors associated with RNA polymerase II. These complexes open up sites on chromatin and initiate transcription.