Api

MRE activator protein

Transcription start

SRNA

polymerase

Various proteins may bind to upstream response elements to stimulate transcription.

Transcription factors

Basal transcription apparatus

4 16.24 Multiple response elements (MREs) are found in the upstream region of the metallothionein gene. The basal transcription apparatus binds near the TATA box. In response to heavy metals, activator proteins bind to several MRE elements and stimulate transcription. The TRE response element is the binding site for transcription factor API. In response to glucocorticoid hormones, steroid receptors bind to the GRE response element located approximately 250 nucleotides upstream of the metallothionein gene and stimulate transcription.

Other response elements found upstream of the metallothionein gene also contribute to increasing its rate of transcription. For example, several copies of a metal response element (MRE) are upstream of the metallothio-nein gene (< Figure 16.24). Heavy metals stimulate the binding of an activator protein to MREs, which elevates the rate of transcription of the metallothionein gene. The presence of multiple copies of this response element permits high rates of transcription to be induced by metals. Two enhancers also are located in the upstream region of the metallothionein gene; one enhancer contains a response element known as TRE, which stimulates transcription in the presence of phorbol esters. A third response element called GRE is located approximately 250 nucleotides upstream of the metallothionein gene and stimulates transcription in response to glucocorticoid hormones.

This example illustrates a common feature of eukary-otic transcriptional control: a single gene may be activated by several different response elements, found in both promoters and enhancers. Multiple response elements allow the same gene to be activated by different stimuli. At the same time, the presence of the same response element in different genes allows a single stimulus to activate multiple genes. In this way, response elements allow complex biochemical responses in eukaryotic cells.

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