Team:PennState/diauxie/intro

A common framework for gene expression at bacterial catabolic operons involves dual regulation by a global regulatory protein and a catabolite-specific regulator (e.g., AraC in the case of expression from promoter PBAD). In E. coli, the cAMP-receptor protein (CRP) acts as a global regulator in which the cAMP-CRP complex typically increases transcription at catabolic promoters in the absence of the “preferred” catabolite glucose. The result is a phenomenon known as diauxie, in which glucose is preferentially utilized in the presence of other sugars, since expression of catabolic pathways for the other sugars is not fully induced. A consequence of this dual control mechanism is that many bacterial promoters commonly used in biotechnology require the absence of glucose for full transcription activation (e.g., the lac and araBAD promoters).

In wild-type E. coli strains, the promoters controlling expression of genes responsible for xylose transport and metabolism are regulated by CRP and the xylose-inducible protein XylR. Our goal in this project is to create and characterize a xylose-inducible but glucose-insensitive gene expression system. This would functionally eliminate a diauxie-type phenotype relating to induction of gene expression from this promoter. In addition to creating a valuable new tool for the part registry, this project has useful applications for biochemical and bioenergy production. Cellulosic biomass feedstocks targeted for biofuel production or other value-added products contain large percentages of glucose and xylose. In industrial fermentations, cells grown on sugars from cellulosic biomass normally consume glucose as their first carbon source. Then, depending on the process, cells either change gene expression to utilize xylose, or the cells and leftover sugars are removed as waste. Both situations lead to inefficiency in production, especially if a continuous growth process is desired. Growing cells on multiple sugars results in a lag time as the cells switch from glucose to xylose metabolism, which complicates and delays the overall process. The gene expression system we are creating could aid in the simultaneous fermentation of mixed sugars, and would have practical applications during the conversion of biomass to ethanol, and for other processes using bacteria for fermentation of low-cost sugar mixtures.

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   <h4><acronym title="Nuclear Hormone Receptor">NHR Biosensors</acronym><br/></h4>
   <dl id="denav">
-  <dd><a href="hbintro" title="Intro to Endocrine Disruption, pseudoestrogens, pthalates, nuclear hormone receptors, and our goals">Introduction</a></dd>
+  <dd><a href="https://2008.igem.org/Team:PennState/NHR/introduction">NHR Introduction</a></dd>
    <dd><a href="https://2008.igem.org/Team:PennState/smartfold/overview">Phthalate Biosensor</a></dd>
    <dd><a href="https://2008.igem.org/Team:PennState/fusion/overview">BPA Biosensor</a></dd>
   </dl>
 </td>
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-<td valign="top" id="pagecontent" width="80%"><span style="font-size: 16pt">Introduction</span>
+<td valign="top" id="pagecontent" width="80%"><span style="font-size: 16pt">Diauxie Introduction</span>
 <hr/>
-<p class="start">There are currently a only few dependable transcriptional induction systems that are readily available for use in <cite>E. coli</cite>  and other bacteria. Some common systems use proteins such as LacI and AraC to sense the level of sugars such as lactose and arabinose respectively. In both these cases the presence of glucose affects the amount of transcription promoted by each operator; in wild-type bacteria the genes downstream of these operators are involved with metabolism of the sugar associated with the control of that gene system. This phenomenon of preferential sugar usage for cell growth is called <strong>diauxie</strong>.</p>
+<p class="start">A common framework for gene expression at bacterial catabolic operons involves dual regulation by a global regulatory protein and a catabolite-specific regulator (e.g., AraC in the case of expression from promoter PBAD).  In <em>E. coli</em>, the cAMP-receptor protein (CRP) acts as a global regulator in which the cAMP-CRP complex typically increases transcription at catabolic promoters in the absence of the “preferred” catabolite glucose. The result is a phenomenon known as diauxie, in which glucose is preferentially utilized in the presence of other sugars, since expression of catabolic pathways for the other sugars is not fully induced.  A consequence of this dual control mechanism is that many bacterial promoters commonly used in biotechnology require the absence of glucose for full transcription activation (e.g., the <em>lac</em> and <em>araBAD</em> promoters).</p>
-<p>In this project our goal is to introduce <strong>a new induction system</strong> that uses xylose as the inducer. To make this system unique, transcription will be activated in the presence of xylose regardless of the presence of glucose in the cell. This will create a novel and useful new way of initiating transcription that isn’t limited to cases where glucose is not a carbon source.</p>
-<p>In addition to creating <strong>a valuable new tool for the part registry</strong>, this project has useful applications in the <a href="http://en.wikipedia.org/wiki/Bioconversion_of_biomass_to_mixed_alcohol_fuels">bioproduction</a> industry, including (but not limited to) conversion of biomass to ethanol using bacteria. Common biomass feedstocks used for production contain a 50:50 mixture of glucose and xylose. When cells are grown on this mixed carbon source, diauxie causes glucose to be used first, then depending on the process xylose is consumed or removed as waste. This leads to inefficiency in production, especially if a continuous process is desired. Separation of xylose from glucose is very costly, and so is the lag time when the cells are switching from glucose to xylose metabolism. <strong>The induction system we are creating would eliminate this problem by eliminating diaxuie, leading to more efficient production by using  both sugars at the same time.</strong></p>
+<p>In wild-type <em>E. coli</em> strains, the promoters controlling expression of genes responsible for xylose transport and metabolism are regulated by CRP and the xylose-inducible protein XylR. Our goal in this project is to create and characterize a xylose-inducible but glucose-insensitive gene expression system. This would functionally eliminate a diauxie-type phenotype relating to induction of gene expression from this promoter.  In addition to creating a valuable new tool for the part registry, this project has useful applications for biochemical and bioenergy production.  Cellulosic biomass feedstocks targeted for biofuel production or other value-added products contain large percentages of glucose and xylose.  In industrial fermentations, cells grown on sugars from cellulosic biomass normally consume glucose as their first carbon source.  Then, depending on the process, cells either change gene expression to utilize xylose, or the cells and leftover sugars are removed as waste.  Both situations lead to inefficiency in production, especially if a continuous growth process is desired. Growing cells on multiple sugars results in a lag time as the cells switch from glucose to xylose metabolism, which complicates and delays the overall process.  The gene expression system we are creating could aid in the simultaneous fermentation of mixed sugars, and would have practical applications during the conversion of biomass to ethanol, and for other processes using bacteria for fermentation of low-cost sugar mixtures.</p>