Topic_8 - Topic 8: Regulation of Gene Expression II...

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Unformatted text preview: Topic 8: Regulation of Gene Expression II Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: You should be able to describe the process of attenuation, using the tryptophan operon as an example. Other examples of regulatory mechanisms that you should understand are quorum sensing, two ­component regulatory systems, and chemotaxis. Realize that these are examples of common mechanisms that are used in many different regulatory circuits within the cell. An elaborate type of regulatory mechanism that some genes use involves interaction of transcription and translation. This mechanism is called attenuation. Regulation by attenuation occurs after the initiation of transcription, but before transcription is completed. This regulation does not influence the rate of transcription initiation. Typically, if translation of a particular transcript is able to occur, then further transcription of that gene will be terminated after initiation. The model for this mechanism is the tryptophan operon of Escherichia coli. Operons that use attenuation are often amino acid biosynthetic operons, for reasons that will soon become obvious. Attenuation of the tryptophan biosynthetic operon involves a translated leader sequence immediately following the operator (Figure 9.26). The leader sequence contains two tandem tryptophan codons. • If tryptophan is abundant, it will be incorporated into the leader peptide chain from the corresponding aminoacyl ­tRNA. If tryptophan is scarce, the tryptophan aminoacyl ­tRNA will be rare, and the leader peptide will not be completely synthesized. • If the leader peptide is fully translated, then transcription of the downstream genes in the tryptophan operon will be terminated. How does translation of a transcript affect further transcription of that same transcript? First, remember that in prokaryotes transcription and translation are coupled. So the 5' end of a transcript can be translated before synthesis of that transcript is complete. As shown in Figure 9.27, the tryptophan operon transcript is subject to strong intramolecular base ­pairing. If translation of the leader peptide does occur, then the translating ribosome is able to continue along the transcript, and blocks the formation of the antitermination stem ­loop structure. The termination stem ­loop structure can then form, and this causes the transcript to be released by the RNA polymerase. If translation of the leader peptide does not occur, because of tryptophan starvation, then the ribosome pauses, and blocks the formation of the termination stem ­loop structure. Transcription can then continue. The tryptophan operon is also subject to regulation by repression at the promoter. The attenuator control acts to fine ­tune the level of expression given by repression control. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld In many cases, the regulation of several genes in an organism will be influenced by a particular environmental change. There are many such global control systems (Table 9.3). It has been established that bacteria are able to sense the cell density within their population. This is called quorum sensing, and it is mediated by molecules called acylated homoserine lactones (AHL), which are produced by the bacteria. The higher the population, the higher the concentration of AHL. The AHL molecules are able to combine with an activator protein, and activate many different genes that are subject to regulation by population density. Another means of global control is the use of alternative sigma factors. Remember that transcription initiation of most genes is carried out by sigma 70, but some promoters are recognized by other sigma factors, such as sigma 32 (heat shock) or sigma 54 (nitrogen regulation). In the case of heat shock, sigma 32 protein is normally degraded very quickly after it is synthesized, but when the temperature rises, the sigma 32 protein is stabilized, and can therefore promote initiation of transcription at sigma 32 promoters. One of the most widespread regulatory mechanisms that bacteria use to respond to changes in their external environment is the use of two ­component regulatory systems (Figure 9.14). As implied by the name, these systems typically consist of two proteins, one of which, the sensor kinase, senses the stimulus, and transduces the signal to the response regulator, which then regulates transcription. The way that the signal is transduced is through phosphorylation events. The sensor, which is usually a transmembrane protein, is phosphorylated in response to the presence of the signal. The phosphorylated sensor is then able to transfer the phosphoryl group to the response regulator. The response regulator is only able to regulate transcription when it is phosphorylated. Regulation may be by either activation or repression, depending on the system. Some examples of two ­component regulatory systems in E. coli are listed in Table 9.1. Chemotaxis is also carried out by a series of signal transduction events (Figure 9.15). The presence of attractant or repellent is first sensed by transmembrane sensory proteins called methyl ­accepting chemotaxis proteins (MCPs). The MCPs interact with the cytoplasmic protein CheW, and modulate the level of autophosphorylation of the CheA sensor kinase (attractants decrease the level of phosphorylation, repellants increase the level of autophosphorylation). The phosphorylated CheA (CheA ­P) transfers the phosphoryl group to the response regulator CheY. CheY ­P differs from other response regulators in that it does not influence transcription of a gene, but rather determines the direction of flagellar rotation. CheY ­P causes clockwise rotation, which means that the cells will tumble. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld There is another level of regulation of chemotaxis, adaptation, and this involves methylation. The protein CheR is able to methylate the MCPs. Another protein, CheB, is phosphorylated by CheA ­P, and CheB ­P is able to demethylate the MCPs. Thus, in the presence of a continually high level of attractant (resulting in lower level of CheA ­P, CheY ­P and CheB ­P), the level of methylation will increase because of lack of CheB ­P mediated demethylation. The level of methylation of the MCPs will affect their sensitivity to the attractant or repellant. Fully ­methylated MCPs are not able to respond to attractant, resulting in, eventually, the phosphorylation of CheA and subsequent phosphorylation of CheB, and demethylation of the MCP. ...
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