Overview & Purpose
Escherichia coli, commonly referred to as E. coli, has many different strains. The most commonly known serotypes of these bacteria can cause serious food poisoning or even fatality in humans. However, most strains are completely harmless. These strains are usually found in the gut of the host and help by producing K2 and helping with digestion. The presence of these bacteria is very beneficial for it helps to prevent pathogenic bacteria from being present in the intestine.
The Lac switch that we have created in the genetic coding of E. coli bacteria produces a glowing blue color that initially runs off of glucose and eventually runs off of lactose. With this technology, we can create a glow stick that can be used in emergency kits that will provide light in dire situations. By using a non-harmful strain of E. coli, we can create an environmental conscious and biodegradable glow stick that will not cause harm to the surroundings.
This technology will prove to be very helpful for hunters or those who are outdoors for they will not have to worry about disposing of their light source. Used like a regular glow stick, the different components of the device will remain separated and will be mixed together to produce light once a certain amount of force is applied.
Basic Components of a Lac Operon
Natural Lac Operon with Various Parts [1
The lac operon itself is a set of genes found in certain bacterias' DNA that is required for the transport and metabolism of lactose. Most commonly found in Escherichia coli, the operon was the first example of a group of genes under the control of an operator region to which a lactose repressor (LacI) binds.
The Lac operon functions as a single transcription unit and in its basic form comprises of an operator, a promoter, and one or more structural genes such as a regulator or terminator that are transcribed into one polycistronic mRNA. Typically, the structural genes include LacZ, LacY, and LacA.
- LacZ encodes β-galactosidase, an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose.
- LacY encodes β-galactoside permease, a membrane-bound transport protein that pumps lactose into the cell.
- LacA encodes β-galactoside transacetylase, an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.
"Only LacZ and LacY appear to be necessary for lactose catabolism" .
When the bacteria are transferred to lactose-containing medium, allolactose (which forms when lactose is present in the cell) binds to the LacI repressor, inhibits the binding of the repressor to the operator, and allows transcription of mRNA for enzymes involved in lactose metabolism and transport across the membrane as seen in the image.
The main idea is that E. coli (the most common medium when investigating the Lac operon) conserves its resources by not making many Lac proteins when other more easily-accepted sugars, such as glucose, are available . This was tested by Jacques Monod during World War II. He tested the combinations of different sugars for E. coli and discovered that when the bacteria are grown with glucose and lactose, glucose would get metabolized first during the bacteria's growth phase I and then lactose during growth phase II.
This means that if glucose and lactose are available for the cell, transcription will occur but at a slow rate. Obviously, if there is no lactose at all, nothing will be transcribed. As long as lactose is available, transcription will happen as the LacI repressor is never binded to the operator. Thus, when these Lac proteins are made with the presence of lactose, the lac gene and its derivatives can be used to trigger a color change within the cell. Once glucose is used up, lactose acts as the power source, and the lac operon can truly act as a reporter gene. As in the case for our group, the lac operon device contained the necessary promoters, ribosome-binding sites, terminators, a LacI repressor, a cyano fluorescent protein, and a vector backbone based on the Type IIS assebmly strategy and would turn a bright cyan color when exposed to lactose. This "switch" function can have a multitude of possibilities, and one of these uses is focused on in the page.
Design: Our genetic circuit
OUR GENE SWITCH:
<tab>pSB1A3-1 is a high copy number plasmid. The replication origin is a pUC19-derived pMB1 (copy number of 100-300 per cell). The terminators bracketing pSB1A3 MCS are designed to prevent transcription from inside the MCS from reading out into the vector.
Building: Assembly Scheme
Testing: Modeling and GFP Imaging
A LAC SWITCH MODEL
We used a previously published synthetic switch, developed by Ceroni et al., to understand how our system could potentially be modeled and simulated. The graphic to the left depicts the relationships between the parameters of the system using a network diagram illustration.
In order to approximate the behavior of this set-up, a mathematical model can be developed based upon the relationships between different parameters. These relationships can be expressed in mathematical terms using numbers that relate to the system, including creation or decay rates, concentrations, or various constants. The Ceroni et al. model and the network diagram illustration use the following table of terms in their representation of the Lac switch.
Lac Switch Model: Important Variables and Parameters
|| IPTG concentration
|| GFP protein concentration
|| free LacI molecules
|| LacI molecules bound to IPTG
|| mRNA molecules of GFP
|| mRNA molecules of LacI
|| free Repressor/Reporter plasmids
|| Repressor/Reporter plasmids bound to LacI molecules
|| Repressor/Reporter plasmids bound to induced LacI molecules
|| number of Reporter plasmids per cell
|| number of Repressor plasmids per cell
|| protein degradation rate
|| mRNA degradation rate
|| GFP rate of synthesis
|| LacI rate of synthesis
|| GFP transcription rate
|| LacI transcription rate
|| equilibrium binding constant of the LacI-Ox complex
|| equilibrium binding constant for the binding of induced LacI molecule to the Ox operator sequence
|| equilibrium binding constant for binding IPTG-LacI
|| time constant of LacI binding to operator sequences
|| time constant of induced-LacI binding to operator sequences
|| time constant of LacI-IPTG binding
AN INTERACTIVE MODEL
We used a model of the natural Lac operon to understand how changing the parameter values changes the behavior of the system. By changing the initial concentration of input (IPTG in this case), we were able to estimate the threshold that produces an "on" state in the system. Initially, the code had the concentration at 0.32 which is seen in the β-galactoside (Bgal concentration) vs. time plot (fig. 1).
Figure 1: Original Bgal Concentration vs. Time with I = 0.32
This value was changed again to 0.25 in determining the threshold that produces this "on" state (fig. 2).
Figure 2: Bgal Concentration vs. Time with I = 0.25
After proceeding to go up and down with these a values, a threshold was indeed found where the concentration of IPTG is about 0.064 (fig. 3).
Figure 3: Bgal Concentration vs. Time with I = 0.064
COLLECTING IMPERICAL VALUES TO IMPROVE THE MODEL
We explored how one technique, imaging via microscopy could be used to determine the production rate of an output protein, in this case GFP in yeast, could be used to determine a "real" value for maximum GFP production rate under our own laboratory conditions.
- show plot of data and discuss outcome.
- include some of the pictures of the raw data
- wrap up section to explain how the curves could be improved
Ideally, the GFP production rate measured by this method could be entered as a value for [which parameter] in the Ceroni et al. model.
SUPPORTS & UNDERSTANDS
DOES NOT SUPPORT & UNDERSTANDS
SUPPORTS & DOES NOT UNDERSTAND
DOES NOT SUPPORT & DOES NOT UNDERSTAND
- My name is Emily Byrne, and I am a student majoring in biomedical engineering. I am taking BME 494 because ###. An interesting fact about me is that ###.
- My name is Sarah K. Halls, and I am a student majoring in Biomedical Engineering. I am taking BME 494 because I enjoy cell and tissue Engineering work and hope to start my career in this field of study. An interesting fact about me is that I did an internship at Harvard University working on cell patterning.
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- Benno Muller-Hill. The Lac Operon. Walter de Gruyter. ISBN:3110148307
- Ceroni F, Furini S, Giordano E, and Cavalcanti S. Rational design of modular circuits for gene transcription: A test of the bottom-up approach. J Biol Eng. 2010 Nov 11;4:14. DOI:10.1186/1754-1611-4-14 |