"
Team:Alberta/Project
From 2012.igem.org
|
|
The end goal of our project is to create spatial color patterns using bacteria, such as a color wheel and a rainbow. Not only would this give an attractive end result, it would also demonstrate a fine level of control over gene expression, as it requires spatial control over several colors simultaneously. Patterned through spatial gradients of the inducers IPTG and ATC, using established red, yellow, and blue proteins as outputs, we needed to turn on and off expression of these genes as a function of inducer level. Induction of color expression as the inducer level increases is achieved via a typical inducible promoter system, with the repressor expressed elsewhere on the plasmid under a weak promoter. To expand the dynamic range of this system, we developed a means of adjusting gene expression through plasmid copy number control, achieved by placing the RNA II gene in the pMB1 replication origin under inducible control. At low levels of inducer, copy number increases with inducer, as greater RNA II expression leads to increased plasmid replication. Note that this mechanism makes plasmid replication dependent on the presence of inducer, effectively creating a new safety switch for genetically engineered organisms which might enter the environment.
A full color spectrum may be made by mixing red, yellow, and blue in varying proportions. From part sequences in the Registry, we selected the the blue chromoprotein amiCFP (K592010), the yellow chromoprotein amilGFP (K592009), and the classic red fluorescent protein (RFP, E1010). These parts were selected due to their excellent presentation by the 2011 Uppsala iGEM team. We altered the sequences to remove KpnI sites, for convenience during cloning and assembly, designed custom ribosomal binding sites for each open reading frame using the Salis RBS calculator (https://salis.psu.edu/software/) to give consistent medium-high expression levels (designed transcription initiation rate 50k), ordered the sequences synthesized as gBlocks (IDT), and assembled them. Promoters were selected from the Anderson collection of constitutive sigma 70 promoters of various strengths. These promoters were trimmed outside the -10 and -35 boxes, and the bases in the interior region have been varied to limit recombination when multiple promoters are used in the same system. For details on the promoters, see our Parts page
Unfortunately, initial assemblies did not produce color. We therefore modified our parts as follows: for RFP and amilGFP, the engineered RBSes were replaced with B0034, and also a change of base strain. TG1 produces larger, but less saturated colonies than TOP10, however the color develops with time. Factors which lead to additional color expression lead to diminished cell growth, thus best color was achieved with moderate promoter strength.
Since we were uncertain if the promoter and RBS regions were too strong (leading to toxicity, poor cell growth and poor color) or too weak (leading to little protein generation and poor color), we tried several variations, looking for the most saturated colony colors. The resulting color cassettes have been submitted as BBa_K879311 (red), using promoter 3 and the B0034 RBS, BBa_K879313 (yellow), also using promoter 3 and the B0034 RBS, and BBa_K879222 (blue), using promoter 2 and the RBS engineered for amilCP.
Fig ###. Colour quality. Shown is the range of colour development for our submitted parts, [http://partsregistry.org/wiki/index.php?title=Part:BBa_K879222 BBa_K879222] containing amilCP (blue), [http://partsregistry.org/wiki/index.php?title=Part:BBa_K879313 BBa_K879313] containing amilGFP (yellow under visible light/green under UV light) and [http://partsregistry.org/wiki/index.php?title=Part:BBa_K879311 BBa_K879311] RFP (red) under different growth and light conditions. Each pigment gene is driven from a constitutive promoter. Complete descriptions can be found under Parts. The pellet colours shown in the bottom-right panel are derived from the overnight cultures shown above. Note that amilCP is not a fluorecent chromophore- too bad!
For our first try at colour regulation we placed the RFP gene onto pS1C3 under the control of the synthetic LacI inducible promoter previously described by Sidney et. al (2007). Here the LacI operator has been trimmed to 17 bp that sits between the -10 and -35 region of the promoter. The gene for LacI was positioned downstream of RFP using the standard biobrick assembly method. LacI and was expressed constitutively from a moderate strength promoter (Pr-3 see xxx). When we examined colour expression of this construct in either liquid culture or as colonies on plates, the results were very disappointing. In the presence of the recommended concentrations of IPTG (0.1mM) only pale pink was observed. The colour was only slightly improved in the presence of 1.0 mM IPTG! A closer look at the literature showed that most people interested in repressor-type regulation, place the repressor as a single copy on the E. coli chromosome [Lutz and Bujard (1997), Sidney et. al (2007)]. Since pSB1C3 is a high-copy plasmid we hypothesized that the concentration of LacI in the cell was too high for efficient induction. We therefore devised an easy strategy to make the LacI promoter a lot weaker. The Anderson promoters have two restriction sites between the promoter regions (AvrII and NheI) which we preserved in our promoter designs. Since NheI is unique in our construct, we filled the site in with DNA poymerase. The 4-bp addition that results, increases the separation between the -10 and -35 regions from the preferred 17-bp to the highly unfavoured 21bp separation [Brunner and Bujard (1987), Hawley and McClure (1983)]. In short, the promoter would be flying on only one wing. When we examined colour expression in the modified construct the results in the presence and absence of IPTG were much more noticable. We further characterized our constructs, which we call weak lacI and strong LacI, by measuring the fluorescence of RFP in overnight cultures as a function of IPTG concentration. The results are presented below.
One potential problem that we became aware of through these experiments is that the range of colour intensity observe in the presence and absence of the inducer is much different between liquid cultures and plates. When cells containing the weak lacI construct are streaked onto plates, the contrast is significantly reduced. in the absence of IPTG pink develops over longer times of incubation so that there is only a narrow window for observing white and red. This was a little worrisome since our plan of controlling colours spatially would be based on the colour development of bacterial “lawns” on plates.
Fig ###. Dynamics of colour expression. Panel A. Schematic of the controllable colour cassette used in these experiments. RFP and LacI are contained on the same plasmid as shown. LacI is constitutively expressed from a very weak promoter (described below) or an intermediate strength promoter (Pr-3). It controls RFP by binding to its operator positioned between the RFP promoter elements. The expression of RFP is induced by the inactivation of LacI in the presence of IPTG. The weak promoter was constructed by filling in the 4-bp overhangs of the NheI site to the 5’ side of the -10 region (see Fig. ###) This displaces the -10 region and transcriptional start 4 bp to the right. Panel B. RFP fluorescence from cultures grown to stationary phase in the presence of different IPTG concentrations . The response of cells expressing the weak LacI cassette is indicated in black. The response of cells expressing the strong LacI cassette is indicated in red.
Methods: Fresh colonies containing either the xxx plasmid or the xxx plasmid were inoculated into 5 mL cultures containing chloramphenicol and IPTG at the following exponential series of mM concentrations: 0, 0.001, 0.002, 0.004, 0.008, 0.016, 0.032, 0.064, 0.128. Cultures were grown to stationary phase in a shaking incubator for 20 hours at 37 C. Colour development resulting from RFP expression was measured by fluorescence as follows: Cells were first diluted 10-fold with TE buffer (10mM Tris, 1mM EDTA pH 8.0) in a final volume of 3 mL (the cuvette volume). Measurements were made using a Perkin Elmer luminescence spectrophotometer (LS 50 B), using parameters that were optimized for the highest value (Plasmid xxx; 0.128 mM IPTG) with the help of Justin Fedor (Alberta iGEM alumnist 2009) These parameters are: 1. excitation wavelength (582 nm), 2. emission wavelength (603 nm), 3. filter cutoff (515 nm), 4. Slit widths lex= 10nm, lem= 10nm)
For the longer-range goal of producing a spectrum on an agar plate, one colour would have to fade out as another colour intensifies. We therefore chose an old and popular method for producing spatial gradients of small molecules that entailed placing a high concentration in a well positioned at the centre of the plate and letting it diffuse over time. If the chemical were IPTG we would expect strong colour at the plate’s centre that faded gradually toward the circumference as the IPTG concentration dropped. Below we have described a fast and reliable way for casting the centre well using cylindrical magnets! Figures ### and ### show the detailed casting procedure, and a rough calibration method using a visible dye.
Fig ###. After several false starts using food coloring, we developed the following protocol for making gradient plates: LB agar plates were made using 25 mL of LB agar. A well in each plate was made using two cylindrical neodymium magnets (dimensions 12 mm long x 4mm dia) per well that sandwich the lid at the preferred location, as illustrated. When solidified the magnet is removed by gently lifting off the lid. Gradients were formed by adding 40uL of either 100x IPTG (10mM) or 100x ATC (100mM) to each well and allowing 8 hours at room temperature for diffusion prior to plating. Overnight cultures are diluted to an approximate cell concentration of between 0.5-1.0x105 cells/ml. 1-2x104 cells in 200 uL are plated by swirling 6 ball-bearings (6 mm dia) on the plate surface until completely wetted. The plates are then dried under a tissue culture cabinet and incubated at 37C overnight. Important! Cell densities that exceed those specified above produce thin transparent lawns with poor colour development.
Spatial patterning of gene expression requires, in addition to spatial chemical gradients, genetic elements allowing gene expression to be controlled by those spatial gradients. We opted to work with three common and well-studied repressors: lacI and tetR, which effectively shut down transcription of operons containing the lacO and tetO operator sequences in their promoter, and for which repression can be relieved by addition of the small molecule inducers isopropyl-thio-galactopyranoside (IPTG) and anhydrotetracycline (ATC).
Figure. RFP on IPTG gradient plates
We were disappointed with the results of testing red gradients under central IPTG control. While the results demonstrated the functionality of control, even when completely repressed the colonies would slowly turn pink over the course of a couple of days.
[xxx describe concept. include figure]
Bacterial cultures were grown at 37OC in LB broth (xxx) overnight under selective conditions that favored plasmid maintenance [chloramphenicol (xxx/mL) and either IPTG (xxx) or ATC (xxx)]. As a starting point, the number of viable cells in each culture was determined by spotting 5uL of 10-fold serial dilutions ranging from undiluted to 10-9 (xxx spots/culture) onto LB-agar plates containing Chloramphenicol and either IPTG or ATC at the concentrations cited above. Cell count/mL of culture was determined by counting (or estimating) the number of colonies at the highest resolvable dilution and multiplying by the dilution factor.
Fresh cultures were then made by inoculating one uL of the original cultures into 5 mLs of LB broth under conditions that favored plasmid loss (no antibiotic or inducer) and grown as above. The number of viable cells remaining under non-selective conditions were determined as described above.
- Making the plates. LB agar plates were made using 25 mL of LB agar. A well in each plate was made using two cylindrical neodymium magnets (dimensions xxx) per well that sandwich the lid at the preferred location, as illustrated below. When solidified the magnet is removed by gently lifting off the lid.
- Making the gradients. 40uL of either 100x IPTG (10mM) or 100x ATC (100mM) are added to each well and allowed to diffuse for 8 hours at room temperature prior to plating.
- Plating bacterial lawn. Overnight cultures are diluted to an approximate cell concentration of between 0.5-1.0x105 cells/ml. 1-2x104 cells in 200 uL are plated by swirling 6 ball-bearings (dimensions xxx) on the plate surface until completely wetted. The plates are then dried under a tissue culture cabinet and incubated at 37OC overnight.
- Important! Cell densities that exceed those specified above produce thin transparent lawns with poor colour development.
Fig #. Auto repression: Two plasmids are initially maintained in one cell under selective conditions (chlr+ /Kan+ /IPTG+ / ATC+). Each plasmid carrying either red or blue regulates its copy number through its own repressor. The absence of each repressor’s inducer results in the loss of the plasmid on which it resides. Cross repression: Plasmid maintenance as above. In this case however plasmids work in opposition. The survival of one demands the loss of the other. Inputs/outputs: In either case, cells grown in liquid culture would be expected to produce 1 of 4 colours (white included) depending on the inducer input combinations. Varying the ratio of inducers would likely vary both the hue and value in ways that would have to be determined experimentally.
- Tested three great protein colors into our lab: RFP (red), amilGFP (yellow in visible light), and blue chromoprotein amilCP.
- Designed and tested versions of the common repressor proteins lambda CI, LacI, and TetR which lack the LVA degradation tag
- Designed tested new RBSes for all of these proteins, using the Salis RBS calculator. For amilCP, the new RBS is substantially stronger than the existing Registry part (K592009); for RFP, color expression is substantially worse (E1010).
- Experienced some of the benefits and weaknesses of current in silico design tools (ie RBS calculator fail)
- Developed and tested methods for measuring diffusion constants of small molecules in agar plates, to aid engineering inducer gradient plates
- Tuned control system for inducible gene expression with repressor being generated on same high copy plasmid as controlled gene
- Currently have three separate colors under multiple constitutive and inducible promoters.
- Developed new system for controllable copy number, based on the use of repressible promoter for the RNA II gene in the pMB1 replication origin. We have validated this system with both LacI repression and TetR repression through assessment of colony size and color output intensity.
- All developed parts have been sequenced, validated, and submitted to the Registry.
- We have supplied strains to the UBC, Calgary, and CINVESTAV-IPN-UNAM_MX iGEM Teams.
