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Characterisation of the fluorescent reporter iLOV

Introduction

Prior to selecting explosive degrading genes, a reporter gene that would enable quick feedback on the successful expression of operons was needed. The reporter gene would also be used in a biosensor of explosive compounds. While the Green Fluorescent Protein (GFP) and its variants are the most commonly used reporters on the iGEM database and in the wider scientific community, the protein iLOV was chosen as the reporter for our constructs.

iLOV was introduced to the iGEM database in 2011 by Glasgow University [1], but so far has seen no further characterization. iLOV has a range of advantages over GFP and related florescent proteins (FP) including:

• Recovers spontaneously after photo-bleaching [2]
• Half the sequence length of GFP (Halving DNA sequencing costs) [2]
• Faster maturation of iLOV flurophore than GFP [2]
• Stable over a wide pH range [3]
• Ability to work in anoxic conditions [1]

iLOV's pH compatibility and photo-bleaching resistance are crucial to the success of a biosensor device that would be required to work in a range of soil types and climates. For example some of the most heavily mine contaminated areas in the world are in western Sahara along the Berm, a 1500 mile long minefield built to protect the Morrocan border. Under such conditions an on-site application of the bio-sensor would need to resist photo-bleaching in the harsh Saharan sunlight.

The aim of this study is twofold: the emmission spectra of iLOV will be explored over a range of excitation wavelengths and recovery rates from photo-bleaching will be measured.

Experimental Method

Excitation and Emission Scanning

The scanning of the excitation and emission spectra would occur in two stages. First a broad range scan, 5nm spacing between emission scans, provides the general topography of florescence from the iLOV protein. Then areas around the peaks from the broad range scan would be explored in more detail with 1nm spaced emission scans.

The scans would be made using a TECAN florescence measuring machine. This machine is capable of measuring fluorescence across a 96 well plate, enabling multiple measurements of each iLOV growth culture. The pitfall of the machine's high through put design is that measurements of fluorescence are made by both exciting the protein and detecting emitted photons along the same axis (i.e. directly above the well). This results in photons emitted from the machine reflecting back from the bottom of the well and interfering in emission measurements. As such the TECAN cannot measure emissions from the protein that are closer than about 30nm from excitation wavelengths.

Top10 cells, with and without the iLOV expression plasmid (Part BBa_K660004) were grown overnight in liquid broth. 10 $\mu$l of the overnight culture was added to 190 $\mu$l of minimal media into each well of the 96 well plate. Minimal media removes the autofluorescence observed using Lysogeny Broth (LB).

The cultures where grown at 37$^o$C with constant shaking at 800rpm until they reached an optical density (OD) above 0.5 at 600nm. The Broad range emission scans at excitation intervals of 5nm from 350nm to and emission was measured from 400nm to 700nm in 5nm intervals. To reduce potential photo-bleaching the number of emission reads for each measurement was limited to five and the wells used for each measurements where alternated. Due to reflection from the excitation photons a portion of the measurements must be discounted where the excitation wavelength came within around 30nm of the emission measurement wavelength.

Fluorescence from each well was divided by the OD specific to that well to standardise fluorescence to cell density. The relative fluorescence of the control cells (Top10) was then subtracted from those cells expressing iLOV.

The second set of finer spaced emission scans, were carried out around peak identified . These scans were taken at 1nm excitation and emission intervals between points defined by the prior part of this experiment.

To minimise the risk of photo-bleaching the iLOV protein only three wells where used for each set of measurements and these were alternated over the course of the experiment. For the finer spaced measurements six well were used to improve the accuracy of the result.

Photo-Bleaching of iLOV and GFP

To test the effect of photo-bleaching on iLOV and GFP two measurements were carried out. The first to determine how exposure time to exciting photons affects protein fluorescence, the second will measure the ability to recover and recovery time from prolonged exposure.

Samples were grown as previously stated, in a 96 well plate and for this experiment E. coli expressing GFP was included as a direct comparison.

The measuring conditions from the detailed scan where be repeated, only without changing the excitation wavelength. This was only repeated for iLOV and Top 10 wells. The repeat would allow for any photo-bleaching effect to be observed minus the bias from changing wavelength. Three scans, in quick, succession, were completed at an excitation wavelength of 449nm and emission was measured from 460nm to 550nm in 2nm intervals to increase the exposure time to 4.5 ms per scan.

To determine how fluorescence is effected by exposure time the scan was repeated. Only three wells were scanned at a time, rotating there use to give the protein more time to recover between experiments. The number of reads per measured emission wavelength was decreased to 1, 20$\mu$s integration. This cut the exposure time by a factor of 10. Triplets of scans were then taken with increasing number of reads (up to 20) to increase the exposure time yet further.

The energy of the photons was not changed as the highest energy available bleached the sample only marginally more than 449nm photons. This is shown in figure ********************, the result of preliminary test to determine whether it was best to change the energy of the photons or the exposure time. This test was completed using the same samples used in the intensity experiment, changing the wavelength of the emitted photons and measuring the florescence drop. Using 449nm photons to excite the protein produces the maximum fluorescence output whilst bleaching the sample.

To measure the recovery rate of the iLOV the protein was exposed to exciting photons for 40 ms. The fluorescence of the protein was then measured with 5 20$\mu$s reads at an excitation wavelength of 449nm and emission wavelength of 494nm. This only exposed the protein for 100 $\mu$s at a time, hopefully not affecting the recovery rate greatly. The fluorescence was measured in triplets with 10 second intervals between measurements.

To expose the difference between the effect of photo-bleaching on iLOV and GFP, the two previous experiments were repeated for the E. coli containing a GFP sequence. The emission of GFP at a 449 nm excitation wavelength is higher than that of native Top 10 cells allowing for the measurement of photo-bleaching despite not using the peak excitation wavelength for GFP.

Results, Analysis and discussion

Excitation and Emission Scanning

The fluorescence from the iLOV protein is found using equation 1, \begin{equation} \operatorname{F} = \frac{1}{N}\sum_{i}^{N} \frac{f_i}{O_i} - \frac{1}{M}\sum_{j}^{M} \frac{g_j}{O_j}. (1) \end{equation}

The fluorescence from each of the wells is divided by the OD in that well ($O_{i/j}$). The average fluorescence per OD of the Top 10 cells ($g_j$) is then subtracted from the average fluorescence per OD of the Top 10 cells with iLOV expressing sequences ($f_i$). This leaves only the fluorescence due to the iLOV protein.

Figure 2 shows the emission topography of the iLOV protein in E.coli. It is clear that there is one large peak of emission centered around an excitation wavelength of 440nm which causes the iLOV protein to emit at around 500nm. The plane of zero fluorescence at the top left of figure 2 is caused by reflection from the excitation photons.

Following from this result the area between 420nm and 480nm excitation wavelengths were chosen to be scanned at a higher resolution. The emitted fluorescence would be measured at 2nm intervals between 460nm and 550nm for excitation wavelengths spaced 1nm apart.

Figure 3 shows the result of the higher resolution scan. It can immediately be seen that there is some artificial effect at work. The striation of the results is most probably caused by photo-bleaching, where the exciting photons have changed the conformation of the protein. The altered state of the protein may have a higher or lower rate of reemission of the excitation photons.

The striation of the results invalidates them from determining the peak emission and excitation wavelength as it cannot be determined whether it is the rest conformation of iLOV that is being measured.

Photo-bleaching was observed in the high resolution scan but not in the broad range scan. This may be because during the broad range emission scans the proteins are exposed to exciting photons for 1.8 ms per scan, whilst in the high resolution scan the protein are exposed for 4.5ms per scan. Three scans are taken in quick succession.

To determine whether the effect is indeed artificial, the fluorescence emission at 500nm was plotted across the excitation wavelengths. This can be seen in figure 4. The triplets of successive scans are coloured. Measurements taken from the the broad range scan are plotted for reference.

From figure 4 it can be seen that the first measurement is notably higher than the trend for the next two measurements in each triplet. This would suggest that the protein has been photo-bleached as the emission response decreased after exposure to excitating photons. The difference in fluorescence between the high resolution scan and the broad range scan is greatest at excitation wavelengths of 420nm and 440nm which correspond to measurements taken in the final scan of a triplet. This effect will be explored in more detail in the photobleaching section.

The errors in figure 4 are calculated via equation,

\begin{equation} \delta F = \sqrt{\sum_{i}^{N}\left(\frac{f_i\delta O_i}{O_i^2 N}\right)^2+\sum_{j}^{M}\left(\frac{g_j\delta O_j}{O_j^2 M}\right)^2}, (2) \label{Error} \end{equation}

where $\delta O_i$ is the error in the OD for each well, $f_i$, $g_i$ are the fluorescence measurements from iLOV and Top 10 wells respectively and N, M are the number of wells used.

Due to the striation of the fine-scale results a previous measurement was used to confirm the peak of fluorescence. This result can be seen in figure 5.

This result was not used originally as measurements where carried out in Lysogeny Broth (LB) which has a higher autofluorescence than the minimal media used previously. The higher autofluorescence of the LB resulted in a masking of the fluorescence topography from the Top 10 wells. This resulted in the Top 10 cells without iLOV sequences reading as having a higher fluorescence than those with the iLOV sequence for certain ares of the experimental space. The fluorescence topography of the iLOV protein is still visible despite this and matches the topography observed in minimal media.

The peak excitation wavelength is 449$\pm$1 nm and the peak emission corresponding to this excitation wavelength is 494$\pm$2nm. The peak has a lower apparent fluorescence than previous measurements, this is due to a lower manual gain on these measurements. The manual gain is the factor that the measured intensity is multiplied by. It was lowered for these measurements in an effort to take emission measurements closer to the excitation wavelength. However this was unsuccessful, only resulting in some reflection measurements being read as within the acceptable fluorescence range.

No photo-bleaching was observed in this measurement despite the protein being exposed for 5ms per scan. This is likely due to the greater opacity of LB in comparison to minimal media.

Photo-Bleaching of iLOV and GFP

Summary

could have used this to decrease error in GFP recovery rate \begin{equation} t = 20 \mu \text{s} \frac{\lambda_{\text{GFP}}}{\lambda_{\text{iLOV}}} = 17.6\mu \text{s}, \end{equation}

References

1. 2011.igem.org/Team:Glasgow/LOV2
2. The phtoreversible fluorescent protein iLOV outperforms GFP as a reporter of plant virus infection. Chapman, et al. www.png.org/content/105/50/20038.full
3. Swartz TE, et al. (2001) The photocycle of a flavin-binding domain of the blue light photoreceptor phototropin.

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