qRT-PCR for the relative quantification of specific tRNA-species

Alongside with the generation of a climate-friendly medium, the goal of our project PhyCoVi was to optimize the strain Vibrio natriegens for a potential use in the biotech industry. The optimization is performed on the genomic level to increase the intracellular availability of tRNA species. As a result, the strain’s performance to express heterologous proteins is enhanced.

A method needs to be developed to quantify individual tRNA species specifically to prove the increased expression not only on the protein level.  Multiple methods can be found to quantify non-coding RNA ^{1, 2} or total tRNA concentration ^{3, 4}. Whereas finding a well-established method to quantify single tRNA species specifically is in vain. The only method paper was published in the journal “RNA biology” in 2015 by Honda et al.: “Four-leaf clover qRT-PCR: A convenient method for selective quantification of mature tRNA” ⁵. The authors of this paper removed the amino acid at the 3’ end followed by hybridization and ligation with a DNA/RNA hybrid stem loop creating a “four-leaf clover” shaped appearance of the tRNA ligation product. The stem loop adaptor contained a TaqMan probe binding site. During the qPCR the TaqMan probe was cut by exonuclease function of the used polymerase resulting in emission of fluorescence.

Building on the work of Honda et al. we developed a new and simplified method for relative quantification of specific tRNA species without the necessity of TaqMan probes. Instead using a DNA/RNA hybrid stem loop we used a linear DNA/RNA construct as adaptor.

The first step is to isolate RNA with a length of < 200 nt from cultured V. natriegens cells. Then, the amino acid bound to the 3’ end needs to be removed by a deacylation reaction. This results in a sticky end, where a linear RNA/DNA hybrid adaptor can be ligated, which is complementary to the 3’ end overhang. Although different tRNAs show differences in length and sequence, the last three nucleotides at the 3’ end are the same for all tRNA species. The ligated adaptor contains a binding site for the forward-primer, which is identical for all tRNAs (unspecific primer). We used T4-RNA-ligase 2 that requires ATP. For this reason, a polynucleotide kinase was necessary to carry out a phosphorylation reaction at the 5’ end.

To amplify single tRNA species specifically, we distinguished between two options. First option was using the specific tRNA primer in a reverse transcription to convert the whole tRNA pool to cDNA. Following RNase H digestion results in pure cDNA of the desired tRNA species.

Later the desired tRNA species is amplified during a qPCR by using the specific reverse primer and the unspecific adaptor primer.

During qPCR a DNA-intercalating fluorescence dye (Green DNA dye) allows for relative quantification: Green DNA dye binds to double stranded DNA and absorbs blue light and emits green light. The more double stranded DNA is generated, the higher the resulting fluorescence. And the higher the concentration of the template in the sample the faster the fluorescence exceeds the threshold. The number of cycles at which this happens is called the threshold cycle (C_t). (e.g. if sample A showed a C_t of 8 and sample B showed a C_t of 11, sample A contained 2³ = 8 times more template.)

After running a DNA gel, we noticed that the obtained amplification products did not show the expected length. This may have been a result of distinct secondary structures of the tRNA species: the reverse transcription reaction was performed at 42 °C which is the enzyme’s optimum working temperature. However, this temperature is not high enough to prevent secondary structures or to break them up. Therefore, areas with secondary structures may have been inaccessible for the reverse transcriptase resulting in shorter cDNA fragments.

For this reason, we tested a second option to amplify single tRNA species specifically. A modified polymerase together with the specific reverse primer can be used to amplify the desired tRNA species using RNA as a template. This modified polymerase works at temperatures around 65 °C and can use both RNA and DNA as a template. The reverse transcription reaction is thus not needed as a consecutive step anymore. Moreover, the modified enzyme creates the specific cDNA from RNA directly and the high temperature prevents secondary structures. The relative quantification based on C_t values is the same as in the option described before.

Cloning of tRNA fragments into pSB1C3

The tRNA fragments were synthesized by IDT and amplified by PCR according to the PCR protocol (Protocol_PCR.pdf). The amplified tRNA fragments were validated via agarose gel electrophoresis (Stuttgart--Protocol_Agarose_Gel.pdf). Looking at Figure 1 all tRNA fragments showed a clear band at the desired height.

Figure 1: Amplified tRNA fragments. The tRNA fragments AGA, CGC, TGC, TCC and AGG were amplified via PCR. The PCR products were separated by agarose gel electrophoresis. A 2% agarose gel was prepared and 10 µL were loaded for each probe ((1): AGA, (2): CGC, (3): TGC, (4): TCC, (5): AGG). 3 µL of Hyberladder 1 kb Bioline were loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using MidoriGreen.

In a first step, the tRNA fragments were cloned into the pSB1C3 vector. The pSB1C3 and the tRNA fragments AGA, AGG, CGG, TGC, TCC and a combined tRNA fragment containing all 5 tRNAs were digested using the restriction enzymes XbaI and SpeI.
After purification, ( Protocol_Clean_and_Concentrate.pdf) the digested fragments were ligated using the T4 DNA ligase (see Protocol_BioBrick_Cloning.pdf). The ligated DNA fragments were transformed into DH5α ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with XbaI to gain linear Plasmid and separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Looking at Figure 2 the obtained plasmids showed no insert, only pSB1C3. Also visible is, that despite the digestion, circular, supercoiled and open circular structures of the plasmid are still present. This indicates inefficient digestion by the restriction enzymes.

Figure 2 - Cloning of tRNA fragments into pSB1C3. The pSB1C3 and the tRNA fragments were digested using the restriction enzymes XbaI and SpeI. After purification, the digested fragments were ligated using the T4 DNA ligase. The ligated DNA fragments were transformed into DH5α and subsequently prepared. The obtained plasmids were digested with XbaI before the agarose gel electrophoresis. A 1% agarose gel was prepared and 10 µL were loaded for each probe ((1): AGA, (2): AGG, (3): CGG, (4): TGC, (5): TCC, (6): Combined tRNA fragment). As a control (C) the linear pSB1C3 was loaded. 3 µL of GeneRuler, 1kb Plus DNA Ladder was loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using GelRed.

Since the cloning showed inefficient digestion by the enzymes XbaI and SpeI, it was performed again with the enzymes EcoRI-HF and PstI. The pSB1C3 and the tRNA fragments AGA, AGG, CGG, TGC, TCC and a combined tRNA fragment were digested using the restriction enzymes EcoRI-HF and PstI.
After purification, ( Protocol_Clean_and_Concentrate.pdf) the digested fragments were ligated using the T4 DNA ligase (see Protocol_BioBrick_Cloning.pdf). The ligated DNA fragments were transformed into DH5α cells ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with EcoRI-HF to gain linear Plasmid and separated by agarose gel electrophoresis (Protocol_Agarose_Gel.pdf). The agarose gel revealed no successful cloning as all obtained plasmids showed no insert, only pSB1C3 (data not shown).

Cloning of tRNA fragments into ptRNA_backbone via BioBrick Cloning

In a first step, the self-designed linear ptRNA_backbone from IDT was ligated according to the NEB Ligation Protocol with T4 DNA Ligase (see T--Stuttgart--Blunt_End_Ligation.pdf). Afterward, the ligated ptRNA-backbone was transformed in E. coli DH5a cells (see Protocol_Transformation.pdf). Successfully transformed DH5α cells were selected on LB agar plates containing tetracycline. The next day the circular ptRNA_backbone was prepared from the colonies according to the Plasmid Preparation protocol ( Protocol_Plasmid_Preparation.pdf). The plasmid obtained from the colonies was digested with EcoRI-HF to gain linear Plasmid and separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Looking at Figure 3 all plasmids run at the desired length which corresponds to the length of the ptRNA_backbone 2159 bp.

Figure 3 -Cloning of ptRNA_backbone. The linear ptRNA_backbone fragment from IDT was ligated using the T4 DNA Ligase. The ligated ptRNA-backbone was transformed into DH5α and subsequently prepared. The obtained plasmids were digested with EcoRI-HF before the agarose gel electrophoresis. A 1% agarose gel was prepared and 10 µL were loaded for each probe ((1): ptRNA_backbone gBlock from IDT, (2): colony 2, (3): colony 3, (4): colony 4, (5): colony 5, (6): colony 6). 3 µL of GeneRuler, 1kb Plus DNA Ladder was loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using GelRed.

The ptRNA_backbone and the previously amplified tRNA fragments AGA, AGG, CGG, TGC, TCC and a combined tRNA fragment were digested using the restriction enzymes EcoRI-HF and PstI. After purification, ( Protocol_Clean_and_Concentrate.pdf) the digested fragments were ligated using the T4 DNA ligase (see Protocol_BioBrick_Cloning.pdf). The ligated DNA fragments were transformed into DH5α ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with EcoRI-HF to gain linear Plasmid and separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Figure 4 reveals no successful cloning as all obtained Plasmids show no insert, only ptRNA_backbone.

Figure 4 - Cloning of tRNA fragments into ptRNA_backbone. The ptRNA_backbone and the tRNA fragments were digested using the restriction enzymes EcoRI-HF and PstI. After purification, the digested fragments were ligated using the T4 DNA ligase. The ligated DNA fragments were transformed into DH5α and subsequently prepared. The obtained plasmids were digested with EcoRI-HF before the agarose gel electrophoresis. A 1% agarose gel was prepared and 10 µL were loaded for each probe ((1): AGA, (2): AGG, (3): CGG, (4): TGC, (5): TCC, (6): Combined tRNA fragment). As a control (C) the linear ptRNA_backbone was loaded. 3 µL of GeneRuler, 1kb Plus DNA Ladder was loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using GelRed.

This BioBrick cloning was repeated several times using different EcoRI-HF and PstI stocks. Following transformation in DH5α revealed no successful cloning. The colonies obtained showed no insert in an agarose gel and only ptRNA_backbone. Following transformation in competent Vibrio natriegens cells also revealed no successful cloning (data not shown).

Cloning of tRNA fragments into ptRNA_backbone via Gibson Assembly

Cloning of tRNA fragments into ptRNA_backbone was also performed using Gibson Assembly. Gibson Assembly was conducted according to the protocol Gibson Assembly ( Protocol_Gibson_Assembly.pdf). The Gibson reaction was transformed into competent DH5α cells ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with EcoRI-HF to gain linear Plasmid and separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Figure 5 reveals no successful cloning as all obtained Plasmids show no insert, only ptRNA_backbone.

Figure 5 -Gibson Assembly of tRNA fragments into ptRNA_backbone. Gibson Assembly was performed according to the Gibson Assembly Protocol. The Gibson Assembly reaction was transformed into DH5α and subsequently prepared. The obtained plasmids were digested with EcoRI-HF before the agarose gel electrophoresis. A 1% agarose gel was prepared and 10 µL were loaded for each probe ((1): AGA, (2): AGG, (3): CGG, (4): TGC, (5): TCC). As a control (C) the linear ptRNA_backbone was loaded. 3 µL of GeneRuler, 1kb Plus DNA Ladder was loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using GelRed.

Cloning of tRNA fragments into ptRNA_backbone via Gibson Assembly was repeated with another Gibson Assembly Master Mix and revealed no successful cloning. The colonies obtained showed no insert in an agarose gel and only ptRNA_backbone (data not shown).

Isolation of Vibrio natriegens DSM 759 genome chr.1 and amplification of tRNAs

As an alternative to cloning of tRNA fragments provided by IDT, the tRNAs AGA, AGG, CGG, TGC and TCC were amplified from the Vibrio natriegens DSM 759 genome. Therefore, the Vibrio natriegens DSM 759 genome chr.1 was isolated according to the gDNA Extraction protocol ( gDNA_extraction.pdf). The tRNA fragments AGA, AGG, CGG, TGC, TCC were amplified from the Vibrio natriegens DSM 759 genome via PCR with appropriate primers (see Protocol_PCR.pdf). Looking at Figure 6 all tRNA fragments showed a clear band at the desired height. The tRNA fragments were extracted from the agarose gel according to the gel extraction protocol ( Protocol_Gel_Extraction.pdf).

Figure 6: Amplification of tRNAs from the Vibrio natriegens DSM 759 genome. The Vibrio natriegens DSM 759 genome chr.1 was isolated according to the Protocol gDNA Extraction. The tRNA fragments AGA, AGG, CGG, TGC, TCC were amplified from the Vibrio natriegens DSM 759 genome via PCR with appropriate primers. A 1.5 % agarose gel was prepared and 10 µL were loaded for each probe ((1): AGA, (2): AGG, (3): CGG, (4): TGC, (5): TCC). 3 µL of GeneRuler, 1kb Plus DNA Ladder was loaded as a marker (M). The gel was run at 90 V for 1 hour and stained using GelRed.

Cloning of tRNA fragments (amplified from Vibrio natriegens DSM 759 genome) into ptRNA_backbone via BioBrick Cloning

The ptRNA_backbone and the previously amplified tRNA fragments AGA, AGG, CGG, TGC, TCC from the Vibrio natriegens DSM 759 genome were digested using the restriction enzymes EcoRI-HF and PstI. After purification, ( Protocol_Clean_and_Concentrate.pdf) the digested fragments were ligated using the T4 DNA ligase (see Protocol_BioBrick_Cloning.pdf). The ligated DNA fragments were transformed into competent DH5α cells ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with EcoRI-HF and PstI to release inserted tRNA fragments from the ptRNA_backbone and gain linear plasmid. The DNA fragments were separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Looking at Figure 7 no insert band was visible for the obtained plasmids, only ptRNA_backbone, suggesting no successful cloning.

Following transformation in competent Vibrio natriegens cells also revealed no successful cloning (data not shown).

Cloning of tRNA fragments (amplified from Vibrio natriegens DSM 759 genome) into ptRNA_backbone via NEBuilder HiFi DNA Assembly

The tRNA fragments were previously amplified from Vibrio natriegens DSM 759 genome. The NEBuilder HiFi DNA Assembly was performed according to the Protocol NEBuilder HiFi DNA Assembly (https://2019.igem.org/wiki/images/2/25/T--Stuttgart--Protocols_NEBuilder_HiFi_DNA_Assembly.pdf). Following transformation into competent DH5α cells revealed no successful cloning as no colonies were obtained. Following transformation in competent Vibrio natriegens cells also revealed no successful cloning (data not shown).

Cloning of tRNA fragments (amplified from Vibrio natriegens DSM 759 genome) into ptRNA_backbone via Gibson Assembly

Gibson Assembly was performed according to the protocol Gibson Assembly ( https://2019.igem.org/wiki/images/6/6c/T--Stuttgart--Protocol_Gibson_Assembly.pdf). Due to the orientation in the Vibrio natriegens DSM 759 genome only the tRNA fragments AGG and TGC could be used for Gibson Assembly. The Gibson reaction was transformed into competent DH5α cells ( Protocol_Transformation.pdf). The plasmid obtained from the colonies ( Protocol_Plasmid_Preparation.pdf) was digested with EcoRI-HF and PstI to release inserted tRNA fragments from the ptRNA_backbone and gain linear plasmid. The DNA fragments were separated by agarose gel electrophoresis ( Protocol_Agarose_Gel.pdf). Looking at Figure 8 no insert band was visible for the obtained plasmids, only ptRNA_backbone, suggesting no successful cloning.

Cloning of tRNA fragments into ptRNA_backbone via Gibson Assembly was repeated with another Gibson Assembly Master Mix and revealed no successful cloning. The colonies obtained showed no insert in an agarose gel and only ptRNA_backbone. Following transformation of the Gibson reaction in competent Vibrio natriegens cells also revealed no successful cloning (data not shown).

Media based on algae: first tests determining important substrates

Medium based on LB

To first determine whether the extract of chlorella vulgaris was a substitute for yeast extract, LB medium and medium containing chlorella vulgaris extract instead of yeast extract were produced.

1. Media were produced (Protocol_media_first_experiments.pdf).
2. Media were inoculated with escherichia coli or vibrio natriegens.
3. After the over night culture, the turbidity of the tubes was observed.

Result: Growth of both bacteria was observed in both media.

Medium without tryptone

To determine whether chlorella vulgaris extract was able to substitute tryptone as a medium component, additionally, media containing different concentrations of NaCl chlorella vulgaris extract and yeast extract were prepared.

1. Media were produced (Protocol_media_first_experiments.pdf).
2. Media were inoculated with vibrio natriegens.
3. After the overnight culture, the turbidity of the tubes was observed.

Result: No growth was detectable in media without tryptone.

Autolysis in combination with bead-milling Results

Free amino acid estimation with rFAN assay

Samples from Experiment Cell_extraction_with_autolysis_combined_with_bead-milling.pdf were used for the analysis.

Yeast extract is mostly obtained by autolysis ¹. In autolysis cells digest their own cell compounds with their own enzymes ². The idea was to transfer this commonly used principal on algae. Therefore, C. vulgaris and C. sorokiniana were heated to 50 °C in alkaline or acidic environment for 41 h. To further crack the cell wall, both algae were treated with bead-milling afterwards. To quantify the success of cell wall disruption free amino acids were measured with rFAN-assay.

The yield of free amino acids was set into relation with the amount of biomass used in the experiment (figure 1).

The highest amounts of free amino acids were reached with yeast at pH 12 with 4.85 %. Both algae showed very low yield in free amino acids. The best results showed C. sorokiniana at pH 12. It is possible, that the amount of glass beads and the size of the glass beads were to little, which led to less cell wall disruption. Therefore, amino acids would have been retained within the cells. This would explain the little amounts of free amino acids achieved with this method. Also, C. vulgaris and C. sorokinia have a cell wall, in contrast to yeast ³>. Cell walls are harder to break, than a plasma membrane. This could explain the difference between the yeast samples and the algae samples. Due to the low yield in free amino acids, it was decided to investigate other methods for cell extraction of algae.

Anthrone assay to Determine Soluble Carbohydrate Concentration

Similar to the rFAN assay the anthrone assay is a method to detect free monosaccharides in a liquid. Therefor samples from the experiment Experiments_AnthroneAssay.pdf were analyzed. Hereby a calibration curve with known amounts of glucose is created (Figure 2, left side). This calibration curve creates the possibility to calculate the sugar concentration of the samples (Figure 2, right side).

One can tell from the coloring of the samples in figure 2, that the carbohydrate concentration should differ very slightly between the samples pH3, pH6, bead mill extraction +pH3 and bead mill extraction +pH6. Due to the cloudiness of the control sample, a background corrected optical density could not be determined. Therefore, the coloring scheme served as evaluation for successful carbohydrate determination.

Hereby, bead-mill (RKM) with subsequent autolysis at pH3 was determined to be the method of choice.

CDW correlation of algae Chlorella vulgaris Results

By plotting the measured optical densities against the means of the calculated cellular dry weights, a correlation was obtained. It is shown in the following figure.

The trend line in figure 1 is poorly matching the trend of the measurement points. For this reason, the correlation curve was rejected. For improvement of this experiment, measurements should be performed only by one experimenter to reduce pipetting errors or other handling mistakes. Also the measurements should be taken over a longer time period to gain more trust worthy results.

CDW-OD correlation by dilution Results

In the following table you can see the calculated cell dry weights with the corresponding optical density of the tubes. Some tubes broke during the experiment so corresponding measurements could not occur.

Table 2 -Calculated values of the cellular dry weight in g/l with the corresponding optical density measured at 750 nm.

OD	CDW[g/l]
4.87	0.98
4.41	0.88
4.44	0.91
4.02	0.8
3.92	0.82
3.52	0.68
3.57	0.71
3.03	0.57
2.65	0.49
2.8	0.5
2.43	0.48
2.56	0.46
2.29	0.4
2.28	0.4

The cellular dry weight in g/l was then plotted against the optical density measured at 750 nm. The plot with the corresponding trend line is shown in the following figure.

Figure 1 - Cellular dry weight in g/l is plotted against the optical density measured at 750 nm. The linear fit is shown in blue together with its formula.

The slope of the formula further been used for fast estimation of the CDW by measuring the optical density at 750 nm.