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Tamarin: Principles of Genetics, Seventh Edition
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III. Molecular Genetics
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16. Gene Expression: Control in Eukaryotes
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Control of Transcription in Eukaryotes
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n this chapter, we turn our attention to the control of gene expression in eukaryotes. We concentrate on the roles that chromatin remodeling, speci c transcription factors (transcription activating proteins), and DNA methylation play in determining which genes are expressed at a particular time in a particular cell. We also look at some other possible factors in the control of gene expression: transposons and Z DNA. We then look at the control of gene expression during development, using the fruit y as a model system. A single cell, the zygote, becomes a whole organism through controlled cascades of gene expression, pathways that are highly conserved in evolution and relatively few in number. Finally, we look at cancer cell growth out of control and immunogenetics, the way in which immunological diversity is generated.
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RNA Polymerase DNA Prokaryotes Promoter Transcription
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RNA polymerase II Nucleosomes
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Promoter
CONTROL OF TRANSCRIPTION IN EUKARYOTES
In prokaryotes, an RNA polymerase holoenzyme with its promoter-recognizing sigma factor is generally active, transcribing at high levels; repressors are needed to prevent transcription. In eukaryotes, an RNA polymerase holoenzyme (e.g., RNA polymerase II), with its promoterrecognizing TFIID, is generally not transcribing; it needs access to the promoter, which is usually wrapped around nucleosomes, and it needs speci c transcription factors to become active ( g. 16.1). Thus, although the parts of the transcribing machinery of prokaryotes and eukaryotes are generally similar, the essence of prokaryotic transcription is activity, whereas the essence of eukaryotic transcription is inactivity. In addition, eukaryotes generally do not have operons; however, groups of eukaryotic genes involved in the same pathway or function can be induced simultaneously by having common enhancers that respond to the same speci c transcription factors. Such a group of genes is called a synexpression group.
Chromatin remodeling
Specific transcription factors
Transcription
In prokaryotes, the default condition is active transcription. In eukaryotes, the default condition is no transcription since the DNA of promoters is usually wrapped around nucleosomes and speci c transcription factors are needed to recruit the polymerase holoenzyme. Transcription in eukaryotes is generally initiated when speci c transcription factors bind to enhancer sequences near the promoter, and chromatin is remodeled at the promoter.
Chromatin Remodeling
For transcription to take place in eukaryotes, the DNA must be available for the preinitiation complex to form, with its RNA polymerase and general transcription factors. It appears that DNA wrapped around nucleosomes is often not accessible for the formation of the preinitiation complex, but is available for recognition by transcriptionactivating proteins, also called speci c transcription factors (as compared to the general transcription factors of the RNA polymerase machine; see chapter 10). One model of initiation of transcription by genes whose promoters are wrapped around nucleosomes is for speci c transcription factors to recruit chromatin-remodeling proteins. As we discussed in chapter 15, there are two general classes of proteins that remodel nucleosomes: histone acetyl transferases and ATP-dependent chromatin remodeling proteins such as the SWI/SNF complex in yeast. Thus, the presence of one or more speci c transcription factors can begin the process of transcription by recruiting chromatin-remodeling proteins that allow the RNA polymerase access to the promoter.
Tamarin: Principles of Genetics, Seventh Edition
III. Molecular Genetics
16. Gene Expression: Control in Eukaryotes
The McGraw Hill Companies, 2001
Sixteen
Gene Expression: Control in Eukaryotes
Speci c Transcription Factors
As we discussed in chapter 10, eukaryotic transcription begins with the formation of a preinitiation complex formed by the amalgamation of a group of general transcription factors (such as TFIID in RNA polymerase II formation). Proteins that exert control over transcription at speci c promoters are the speci c transcription factors (see figure 10.24). These proteins generally have two domains: a domain that recognizes a speci c DNA sequence, and a domain that recognizes another protein, such as a protein in the preinitiation complex. Thus, these proteins recognize signals in the vicinity of the promoter of a gene, bind there, and initiate transcription. Currently, we believe that the majority of specific transcription factors act by recruiting the components of the RNA polymerase holoenzyme. Thus, the binding of a speci c transcription factor at a promoter is the first step in the formation of a preinitiation complex at the promoter of a gene. Some transcription-activating proteins also recruit chromatinremodeling proteins. An example of a speci c transcription factor is Dorsal, the product of the dorsal gene in fruit ies, active in development. Dorsal controls the transcription of several genes and at several different levels of protein concentration. The ability to have different effects at different concentrations is extremely important, allowing gradients of the same protein to control the expression of different genes. One gene Dorsal controls is rhomboid, which has three sites in its promoter that Dorsal binds to, initiating transcription. Another gene, twist, also has three sites in its promoter that bind Dorsal, also initiating transcription. However, the rhomboid sites are more ef cient in binding Dorsal; thus, rhomboid is transcribed at lower concentrations of Dorsal than twist is ( g. 16.2). One other signal in the control of transcription that is of current interest is methylation.
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