Learning Objectives
13.1 Overview of Gene Regulation
- Identify several different reasons why a cell would regulate its gene expression.
- Identify the point at which control of gene expression usually occurs.
- Describe the usual action of regulatory proteins.
- List differences in the control of prokaryotic and eukaryotic gene expression.
13.2 Regulation of Transcription in Bacteria
- Account for the elements of positive and negative control in expression of the lac operon in E. coli.
- Explain control of gene expression in the lac operon.
- Explain control of gene expression in the trp operon.
13.3 Regulation of Transcription in Eukaryotes
- Distinguish between the role of general transcription factors and activator and repressor proteins.
- Describe events necessary for Pol II to bind to the promoter.
- Explain how transcription factors can have an effect from a distance in the DNA.
- Describe how transcription factors can be activated by signaling molecules, and the ways in which they can interact with DNA.
- Describe how chromatin structure can affect gene expression.
- Explain the function of chromatin remodeling complexes.
13.4 Regulation of RNA Processing and Translation in Eukaryotes
- Explain how gene expression can be regulated beyond the level of transcription.
- Describe what is meant by ‘alternative splicing’ of mRNA and explain how the discovery of this process lead to the rethinking of Beadle and Tatum’s “One Gene-One Enzyme” hypothesis.
- Explain how small RNAs can affect gene expression.
Chapter Summary
Prokaryotes and multicellular eukaryotes both control gene expression, but for quite different reasons. Bacteria must exploit the resources of a changing environment. If they do not adapt, they die, but maintaining numerous unused enzymes is metabolically expensive. Multicellular eukaryotes must be protected from those changes. The hallmark of multicellular organisms is homeostasis: maintaining a constant internal environment. To ensure this, genes must be transcribed in a specific order over a specific time frame. Transcriptional control and post-transcriptional control are two primary levels of gene regulation. The former is the more common method. Transcriptional gene control is mediated by influencing the binding of RNA polymerase to the DNA helix. An mRNA transcript cannot be produced if RNA polymerase cannot bind to the promoter. Control to stimulate transcription can also be effected, thus facilitating the binding of polymerase and promoter.
Prokaryotes alter expression of genes when their environment changes. A common pattern in prokaryotes is that gene products necessary for certain catabolic reactions are only expressed when the substrate is present. Such systems are said to be inducible. Other gene products necessary for anabolic pathways are only expressed when the cell needs to build that particular molecule. These are referred to as repressible systems. Each system involves regulatory proteins that will bind to the DNA and alter genetic expression, either by initiating expression (positive) or suppressing expression (negative). Repressors, regulatory proteins that exhibit negative control, act as OFF switches. These can be seen in both inducible and repressible systems. In the inducible E. coli lac operon, lactose binds to the regulatory protein and prevents it from halting transcription necessary for lactose metabolism. In the repressible trp operon, tryptophan binds to the regulatory protein allowing for the suppression of expression genes necessary for tryptophan synthesis. Activators, regulatory proteins that exhibit positive control, are ON switches to ensure that transcription does not occur unless a specific activating chemical is present. The E. coli catabolite activator protein (CAP) is a good representation of this system. The lac operon of E. coli combines ON and OFF switches to ensure that (1) the lactose degrading enzymes are not produced when glucose is present – there’s no need for it since glucose is a better food source, and (2) they are only produced when lactose is present – there’s no need to make enzymes if their substrate isn’t present.
Genetic regulation in eukaryotes is much more complicated than what is seen in prokaryotes. In comparing transcriptional control between eukaryotes and prokaryotes, similarities due exist. Regulatory proteins, called transcription factors, must bind to DNA to regulate transcription. Transcription factors can either be basal transcription factors, proteins necessary for recruitment and proper binding of RNA pol II, or specific transcription factors, proteins that alter expression levels depending on specific signals.
Eukaryote gene control greatly depends on the structure of the eukaryotic chromosome. Histones affect gene transcription by physically blocking the promoter with the nucleosome they create. Methylation, once thought to be a primary regulator in vertebrates, helps ensure that once a gene is turned off, it stays off. Post¬transcriptional control is common in eukaryotes. Researchers have found that small RNA molecules seem to interfere with translation directly or the breakdown of the mRNA before translation. The eukaryote primary mRNA transcript is a linear patchwork of coding exons and noncoding introns. The entire sequence is made during transcription, the introns are cut out later. In many cases, the various ways the exons can be spliced back together allows for production of different polypeptides from just one gene. Aside from the importance of gene control, this kind of transcription seems quite wasteful. Only ten percent of all transcribed genes are exons and only half of that ever gets out of the nucleus. It is yet unknown as to whether this is under any kind of selective control. Proteins called translation factors regulate production of polypeptides from the mRNA transcript. Translation repressor proteins can also shut down translation by preventing the attachment of the transcript to a ribosome. Although most mRNA transcripts are very stable, some, like those associated with regulatory proteins and growth factors, are less stable. They possess certain 3’ sequences that make them attractive to mRNA degrading enzymes. This ensures that control by these proteins remains as transitory as it should be.
Virtual Lectures
CAP-inducible operon vs trp-repressible operon
Eukaryotic Transcription Activation
Eukaryotic Transcription Repression
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