This section deals with the regulation of genes that encode proteins and that are transcribed by RNA polymerase II. As already discussed in Ch. 6, several general transcription factors are required for expression of these genes. Expression also requires specific transcription factors that only affect certain genes in response to specific stimuli or signals. Transcription factors may bind to upstream elements in the promoter region or to enhancer elements that lie far away from the promoter.
Typical specific transcription factors share four general properties:
1. They respond to a stimulus which signals that one or more genes should be turned on.
2. Unlike most proteins, transcription factors are capable of entering the nucleus where the genes reside.
3. They recognize and bind to a specific sequence on the DNA.
4. They also make contact with the transcription apparatus, either directly or indirectly.
Some DNA-binding proteins may respond directly to a stimulus, something that is often the case in prokaryotes. In contrast, the transcription factors of higher organisms are often separated from the original signal by several intervening steps. Here, the DNA-binding proteins and their effects on transcription will be considered.
Transcription factors usually have at least two domains, one that binds to DNA and another that interacts with the transcription apparatus. This may be illustrated by using artificial hybrid proteins consisting of the DNA-binding domain from one protein plus the activation domain of another (Fig. 10.02). A hybrid with the DNA-binding domain from a bacterial protein plus the activation domain of yeast GAL4 activator will no longer activate transcription from the original yeast promoter. However, it will activate transcription from an artificial promoter into which the recognition sequence for the bacterial protein has been inserted. The bacterial DNA-binding protein used in these experiments was LexA, which is actually a repressor.
The mediator complex combines m ultiple signals to regulate transcription of genes by RNA polymerase II.
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