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Tamarin: Principles of Genetics, Seventh Edition
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III. Molecular Genetics
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14. Gene Expression: Control in Prokaryotes and Phages
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The McGraw Hill Companies, 2001
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Gene Expression: Control in Prokaryotes and Phages
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plasmid does not control the bacterial operon; the lac operon on the bacterial chromosome will be continually transcribed.The chromosomal operon has a cis-dominant operator mutation that has a constitutive phenotype. Note, too, that only the bacterial chromosome determines the phenotype because the introduced F plasmid has a z allele.
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Other Lac Operon Control Mutations
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Other mutations have also been discovered that support the Jacob and Monod operon model. A superrepressed mutation, is, was located. This mutation represses the operon even in the presence of large quantities of the inducer. Thus, the repressor seems to have lost the ability to recognize the inducer. Basically, the i-gene product is acting as a constant repressor rather than as an allosteric protein. In an is/i merozygote, both operons are repressed because the is repressor binds to both operators. Another mutation, iQ, produces much more of the repressor than normal and presumably represents a mutation of the promoter region of the i gene. In 1966, W. Gilbert and B. M ller-Hill isolated the lac repressor and thereby provided the nal proof of the validity of the model. At about the same time, M. Ptashne and his colleagues isolated the repressor for phage operons. Control of gene expression in phage is discussed later in this chapter.
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Mark Ptashne (1940 ).
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(Courtesy of Dr. Mark Ptashne.)
Structure of cyclic AMP (cAMP). Glucose uptake lowers the quantity of cyclic AMP in the cell by inhibiting the enzyme adenylcyclase, which converts ATP to cAMP.
C A TA B O L I T E R E P R E S S I O N
An interesting property of the lac operon and other operons that code for enzymes that catabolize certain sugars (e.g., arabinose, galactose) is that they are all repressed in the presence of glucose. That is, glucose is catabolized in preference to other sugars; the mechanism (catabolite repression) involves cyclic AMP (cAMP; g. 14.8). In eukaryotes, cAMP acts as a second messenger, an intracellular messenger regulated by certain extracellular hormones. Geneticists were surprised to discover cAMP in E. coli, where it works in conjunction with another regulatory protein, the catabolite activator protein (CAP), to control the transcription of certain operons.
Tamarin: Principles of Genetics, Seventh Edition
III. Molecular Genetics
14. Gene Expression: Control in Prokaryotes and Phages
The McGraw Hill Companies, 2001
Trp Operon (Repressible System)
In the absence of glucose, cAMP combines with CAP, and the CAP-cAMP complex binds to a distal part of the promoter of operons with CAP sites (e.g., the lac operon; see g. 14.4). This binding apparently enhances the af nity of RNA polymerase for the promoter, because without the binding of the CAP-cAMP complex to the promoter, the transcription rate is very low. The uptake of glucose by E. coli cells causes the loss of cAMP from the cell, probably by inhibiting adenylcyclase ( g. 14.8), and thus lowers the CAP-cAMP level. The transcription rate of operons with CAP sites will, therefore, be reduced ( g. 14.9). The same reduction of transcription rates is noticed in mutant strains of E. coli when this part of the distal end of the promoter is deleted. The binding of CAPcAMP to the CAP site causes the DNA to bend more than 90 degrees ( g. 14.10). This bending, by itself, may enhance transcription, making the DNA more available to RNA polymerase. In addition, at some point in the process of initiation of transcription, the CAP is in direct contact with RNA polymerase. This was shown by photo cross-linking studies in which the CAP was treated with a cross-linking agent that bound the subunit of RNA polymerase when irradiated with UV light. For the two proteins to crosslink, they must be in direct contact during the initiation of transcription. Catabolite repression is an example of positive regulation: Binding of the CAP-cAMP complex at the CAP site enhances the transcription rate of that transcriptional
CAP-DNA interaction: model of cap protein and DNA. The cap site has twofold symmetry, like the operator. The cAMP-binding domain is dark blue, the DNA-binding domain is purple, and the cyclic AMP molecules within the protein are red. The DNA sugar-phosphate backbones are shown in yellow, the bases in light blue. DNA phosphates in red (on the double helix) are those whose modi cation interfere with CAP binding. DNA phosphates in dark blue (also on the double helix) are those especially prone to nuclease attack because of the bending of the DNA. (Courtesy of
Thomas A. Steitz.)
unit. Thus, the lac operon is both positively and negatively regulated; the repressor exerts negative control, and the CAP-cAMP complex exerts positive control of transcription.
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