In recent years the ability to expand the genetic code has been made possible by re-coding the amber stop codon, UAG, via the use of modified tRNA synthetase/tRNA pairs. These synthetase/tRNA pairs act together to charge the tRNA with a non-canonical amino acid (ncAA), an amino acid that is not one of the 20 amino acids normally encoded by a codon. While the library of ncAA synthetase/tRNA pairs continues to grow, the properties of each pairing have yet to be systematically characterized using a standardized methodology. The 2014 University of Texas at Austin iGEM Team has developed a method that allows for efficient in vivo characterization, both qualitative and quantitative, of these novel synthetase/tRNA pairs.
Desired properties??? High fidelity? High efficiency of incorporation? We should indicate what we want our kit to test for.
Background
The genetic code is a composition of 20 highly conserved amino acids that are essential to all organisms on Earth. While the genetic code is specific, it is also degenerate, meaning that more than one codon can encode for the incorporation of a specific amino acid. For example, there are six serine codons and three stop codons (called amber, ochre, and opal). By recoding one of the redundant codons, the recoded codon can signal for the incorporation of a non-canonical amino acid (ncAA) rather than the codon's original usage. Of the three stop codons, the amber codon is the least abundant and thus, the easiest and most efficient to recode.
[Maybe a paragraph on the first recoding??? what amino acid tRNA synthetase and tRNA was used...? This could then transition to something from the following paragraph... but the following paragraph need to be shortened. We would rather talk more about ncAA than amberless e. coli. -dennis]
Complications arise when the genetic code is recoded. In a normal bacterium, release factor RF1 is responsible for terminating translation when the ribosome reaches the amber stop codon. To avoid termination at a UAG amber codon, a strain of E. coli was engineered by the Church and Isaacs groups using MAGE and CAGE (ref) to remove all of the amber codons from the genome and knock out the RF1 gene. The resulting strain, called "amberless" E. coli, has its amber codon free to code for any ncAA. During translation, a synthetase with mutations that allow the acceptance of a different amino acid than the wild type charges that ncAA onto a tRNA with the amber codon's anticodon, CUA, when both are present in the cell.
Experimental Design and Method
In order to recode UAG, a synthetase must be mutated to effectively grab onto an ncAA. Various methods of directed evolution are typically used to modify a synthetase such that it can accept and charge a non-canonical amino acid. The library of ncAA synthetases available have ranging levels of reported efficiency and are not well characterized. This year the UT iGEM Team created a test kit designed to characterize the efficiency of any ncAA synthetase/tRNA pair.
Figure 1. pStG contains the specific ncAA synthetase/tRNA pair being tested. pFRYC and pFRY are the ncAA kit reporter plasmids. pFRYC is the control plasmid that yields GFP and RFP expression regardless of synthetase/tRNA pair. pFRY is a nearly identical plasmid with a single difference, the linker between the GFP and RFP sequences contains an amber codon (star) in place of the tyrosine codon found in pFRYC. Thus, the level of GFP expression for pFRY is directly dependent on the efficiency and fidelity of the synthetase/tRNA pair being tested.
The kit consists of a three plasmid system: pStG, pFRYC, and pFRY (Figure 1).
pStG contains the ncAA Synthetase/tRNA pair (St) to be tested as well as a Gentamicin (G) resistance gene.
pFRYC is the control plasmid and contains an IPTG-induced reporter system and a kanamycin resistance gene. The reporter system is composed of RFP and sfGFP fused via a linker sequence between the two.
pFRY is the experimental reporter plasmid and is nearly identical to pFRYC with the exception that its linker sequence contains an amber codon in the middle of the linker, whereas pFRYC contains a tyrosine codon in the same location.
Using The Kit
To use the kit properly, each culture must have two of the three plasmids from above. There must always be a pStG plasmid as well as either pFRYC or pFRY. However, additional controls can be conducted using only one of the three plasmids.
Figure 2 Schematic demonstrating the gene expression of the kit plasmids under different growth conditions in the presence of IPTG. A) pFRYC (+/-) pStG, (+/-) ncAA. B) pFRY (-) pStG, (+/-) ncAA. C) pFRY (+) pStG, (-) ncAA. D) pFRY (+) pStG, (+) ncAA. The above results represent the "perfect" tRNA synthetase/tRNA pair: one that has 100% fidelity and 100% efficiency. Actual synthetase/tRNA pairs will occasionally misincorporate an incorrect amino acid, and these pairs will not always be perfectly efficient conducting every step in the process of reading through an amber codon.
A) In a cell containing pStG and pFRYC, the ribosome will translate the RFP reporter, linker, and sfGFP, producing red and green fluorescent proteins that result in visible yellow fluorescence (Figure 2A). This happens because the reporter contains no amber codon, and thus does not require a ncAA synthetase/tRNA pair.
B) If a cell contained only pFRY, without pStG, then the ribosome will translate the RFP and terminate at the amber codon on the linker producing a red fluorescence (Figure 2B). This occurs because without the pStG, the amber codon is not recoded to allow for ncAA incorporation.
C) In a cell containing pStG and pFRY, there are multiple possible outcomes, depending on what is present in the culture. In the absence of ncAA, the ribosome will translate the RFP and terminate at the amber stop codon on the linker producing a red fluorescence (Figure 2C). This occurs because while the codon has been recoded, there is no ncAA and without ncAA there can be no incorporation at the amber codon.
D) In a cell containing pStG and pFRY, there are multiple possible outcomes. In the PRESENCE of ncAA, the ribosome will translate the RFP, then it incorporates the ncAA at the amber codon in the linker, and finally it proceeds to translate the downstream sfGFP reporter, producing a visible yellow fluorescence (Figure 2D). This occurs because the synthetase found in pStG can covalently attach the ncAA to the tRNA gene present in pStG. This "charged tRNA" is then present during translation, allowing incorporation of the ncAA into the reporter protein.
However, the above scenarios assume that the synthetase/tRNA pair function efficiently and with high fidelity.
Actual synthetase/tRNA pairs sometimes have low fidelity, leading to the incorporation of an incorrect amino acid. Such a pair would not give results that mimic Figure 2C, expression of only RFP. Rather, in the absence of ncAA, such a pair would give results similar to Figure 2D, full expression of RFP and sfGFP.
Actual synthetase/tRNA pairs can also sometimes have low efficiency. As these are artificially selected pairs, sometimes they do not function as well as natural synthetase/tRNA pairs. In such a case, the amount of sfGFP translated in Figure 2D might be significantly lower than expected.
An ncAA synthetase/tRNA pair was cloned into pStG and transformed into pFRYC amberless E. coli. and pFRY amberless E. coli. Other necessary control strains include RFP amberless E. coli (RFP control), sfGFP amberless E. coli (GFP control), amberless E. coli (cell background control), and LB media supplemented with ncAA (media background control). An overnight culture of each strain was grown in LB with the appropriate antibiotics at 37ºC and 225rpm. 10 mL of media with the appropriate antibiotics was inoculated with 100 µL of overnight culture and allowed to grow in the same conditions until the culture density was ~0.2-0.3 OD, or ~3 hours. The 10 mL culture was split between 4 different sterile test-tubes, 2 mL of culture per tube. The conditions of the four test tubes were as follows:
-IPTG,-ncAA
-IPTG,+ncAA
+IPTG, -ncAA
+IPTG, +ncAA
IPTG stock solution was made at 1000X concentration (why is this here?) and the ncAA was added to yield a concentration of 1 mM. Sterile deionized water was added in the place of ncAA and IPTG as a control (?). Once the controls, IPTG, and the ncAA were added appropriately, the cultures were allowed to grow to ~0.5 OD. 70 µL of each culture condition and control culture was added to a separate wells in a transparent 96-well plate for fluorescence and OD readings in a microplate reader.
[ADD MORE DETAILS LATER?]
Results and Data
Accuracy of Incorporation
Figure 3. Graph showing the level of GFP fluorescence relative to RFP fluorescence.
The accuracy of each ncAA sythetase/tRNA pair was measured by comparing the GFP production of pStG/pFRY strains in (+/-) ncAA conditions. In the absence of a non-cannonical only RFP should be translated. Translation is expected to terminate between RFP and GFP at the amber stop codon (UAG) on linker sequence on pFRY. Alternatively, translation should continue through the linker in the presence of a certain non-cannonical due to its incorporation at UAG. In this case, RFP and GFP should both be translated. The fluorescence of RFP and GFP is detectable with fluorometer.
Strains containing pFRY and pStG plasmids were grown in the presence and absence of the corresponding non-cannonical amino acid. The fluorescence of RFP and GFP readings of each culture were recorded at a culture density of 0.5 OD 600. GFP values of each culture were normalized to RFP values for analysis. The normalized GFP values were compared between cultures grown in the presence of ncAA and cultures grown in the absence of ncAA. Assuming (+) ncAA cultures showed the higher GFP fluorescence, the greater difference in GFP fluorescence between (+/-) ncAA cultures, the more accurate the ncAA synthetase/tRNA pair. Most ncAA synthetase/tRNA pairs resulted in higher GFP fluorescence in the presence of ncAA than in the absence of ncAA, with the exception of 3-aminotyrosine. This suggests that 3-aminotyrosine synthetase/tRNA pair is the least accurate pair from the library tested.
Incorporation Value
Figure 4. May need to consider a different naming convention or the cultures. Possibly AY-C and AY-E for Control and Experimental?? The fluorescence and OD600 readings of each culture were used to calculate a value for incorporation efficiency of each synthetase. Need to add an explanation of how these values were calculated
The fluorescence and OD600 readings of each culture were compared to evaluate the level of incorporation by each synthetase/tRNA pair (pStG). The synthetase/tRNA pairs that showed the highest level of ncAA incorporation include 3-iodotyrosine, 4-azidophenylalanine, 3-nitrotyrosine, and ortho-nitrobenzyltyrosine in decreasing order. Among the ncAAs tested, ONBY consistently slowed the growth rate of the culture significantly suggesting possible toxicity to the cell however ncAA incorporation remained high. Synthetase/tRNA pairs that showed the lowest levels of ncAA incorporation include 3-aminotyrosine, L-dopa, tyrosine, and 4-cyanophenylalanine.
Discussion
Due to the nature of the ncAA Kit Test, in vivo influences will affect certain data and results. This knowledge is very useful by providing a broader and biological perspective of in vivo perspective of cellular health.
In general, the presence of ncAAs slowed the growth of Amberless E. Coli. Among the library of tested non-cannonicals, ONBY and 3-iodotyrosine slowed growth the most, especially ONBY. For consistency, cultures grown with ONBY required nearly twice the amount of time before reaching the appropriate OD 600 for measurement. Despite these inhibitions, the ONBY synthetase/tRNA pair proved to be one of the most efficient pairs tested.
Influences that resulted from ncAA addition that were accounted for and should be noted include: light-sensitivity, oxidation, and interference with fluorescence readings. Certain ncAAs were protected from light due to their light-sensitive molecular structures such as ONBY and AZY. These ncAAs were prepared in a dark room and wrapped in foil. All cultures were grown in a foil-wrapped incubator for consistency. Some ncAAs are more prone to oxidation specifically as L-dopa. When oxidized, the solution turns black. To prevent oxidation, each ncAA was prepared the day of the test for consistency. By this method, the oxidation of L-dopa did not occur quickly enough to have an affect on the data. Most non-canonical solutions were transparent once prepared with the exception of 3-nitrotyrosine. The yellow-orange tint of 3-nitrotyrosine solution was accounted for by measuring the fluorescence of the 3-nitrotyrosine/media solution.
Conclusion
ncAA Table
ncAA
Molecular Structure
Molecular Weight (g)
Soluble in
Other Notes
tyrosine (canonical)
181.19
Soluble in water Heat to 70°C and vortex to dissolve Must stay warm to remain in solution
Stock Concentration: 10mM Concentration in Culture: 1mM
3-aminotyrosine
287.14
Soluble in water Heat to 70°C and vortex to dissolve
Stock Concentration: 10mM Concentration in Culture: 1mM
3-nitrotyrosine
226.2
Soluble in water Heat to 70°C and vortex to dissolve
Stock Concentration: 10mM Concentration in Culture: 1mM
3-iodotyrosine
307.09
Soluble in water Heat to 70°C and vortex to dissolve
Stock Concentration: 10mM (.122g in 10mL H20) Concentration in Culture: 1mM
L-3,4-dihydroxyphenylalanine (L-DOPA)
197.1879
Soluble in water Heat to 70°C and vortex to dissolve
Stock Concentration: 10mM Concentration in Culture: 1mM
o-(2-nitrobenzyl)tyrosine (ONBY)
316.308
Soluble in 50% DMSO in Water Heat up to 70°C and add NaOH to dissolve
Stock Concentration: 50mM Concentration in Culture: 1mM Light Sensitive
4-azidophenylalanine (AzF)
206.204
Soluble in 10% DMSO in Water Requires heating and overnight shaking to dissolve
Stock Concentration: <10mM (.02g in 10mL) Concentration in Culture: 1mM Light Sensitive
4-cyanophenylalanine (CNF)
190.2
Soluble in water Heat to 70°C and vortex to dissolve
Stock Concentration: 10mM Concentration in Culture: 1mM
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