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Silicatein gene with strong RBS

Introduction

Silicatein, originating from the demosponge Suberites domuncula, catalyzes the formation of polysilicate. This biobrick contains the short version of the silicatein gene, according to Müller. et al [1][2]. The coding sequence in this BioBrick is set downstream of strong RBS BBa_B0034.

Sequence and Features

Usage and Biology

Silicatein is an enzyme natively found in demosponges and diatoms, where it catalyzes the condensation of silica to form the typical skeletal elements. Here, we use the enzyme to create a polysilicate layer around the host organism E. coli (Figure 1). This biobrick was expressed under the control of an inducible promoter (Lac-promoter), to do so it was cloned in a backbone containing the promoter and all machinery necessary for it to work. This backbone was obtained from pBbS5a-RFP, a gift from Jay Keasling (Addgene plasmid # 35283) [5].

Characterization

In order to characterize the formation of a polysilicate layer around E. coli, we performed the following experiments.

• Rhodamine 123 staining
• Growth study

Staining with Rhodamine 123

In this experiment we imaged the silicatein expressing cells with a fluorescence microscope, after treating them with a fluorescent dye. The fluorescent dye Rhodamine 123 (Sigma) has shown to bind specifically to polysilicate [3]. Cells were stained according to the protocol based on Li et al. and Müller et al. [3][4]. Rhodamine 123 was excited with a wavelength of 395 nm.

We cannot distinguish a clear difference between silicatein and the negative control. The entire medium is fluorescent, which causes overexposure of the camera at high excitation intensity. This might mean that the Rhodamine 123 is not specifically located at the cell walls, but still dissolved in the medium. We do see some fluorescence localized at the cells, but the difference between the fluorescence of the medium and the cells is much smaller than we observed for OmpA-Silicatein (BBa_K1890002). Therefore, we cannot conclude that the strains transformed with this silicatein plasmid are able to synthesize a polysilicate layer around the cell.

Viability

Since the silicatein expressing cells are to cover themselves in polysilicate, their nutrient supply might be limited by diffusion, which can eventually result in cell death. To investigate whether this is indeed the case, a growth study was performed (Figure 3). Cells were grown overnight in selective LB. They were transfered to fresh medium and grown until in exponential phase. Then IPTG was added to induce expression. After a subsequent incubation of three hours, the medium was supplemented with silicic acid as substrate for silicatein. During the following five hours samples were taken, of which a 10^-6 dilution was plated on selective LB plates. Colony forming units (cfu) were counted the day after. As a negative control, cells expressing another type of silicatein BBa_K1890002, not supplemented with silicic acid were used.

This figure suggests that either the polysilicate layer inhibits nutrient diffusion into the cell or the sodium silicate has a detrimental effect on growth.

References

[1] Müller, W. E. G., Engel, S., Wang, X., Wolf, S. E., Tremel, W., Thakur, N. L., … Schröder, H. C. (2008). Bioencapsulation of living bacteria (Escherichia coli) with poly(silicate) after transformation with silicatein-α gene. Biomaterials, 29(7), 771–779.

[2] Müller, W. E. G. (2003). Silicon biomineralization.

[3] Li, C. W., Chu, S., & Lee, M. (1989). Characterizing the silica deposition vesicle of diatoms. Protoplasma, 151(2-3), 158–163.

[4] Müller, W. E. G., Rothenberger, M., Boreiko, A., Tremel, W., Reiber, A., & Schröder, H. C. (2005). Formation of siliceous spicules in the marine demosponge Suberites domuncula. Cell and Tissue Research, 321(2), 285–297.

[5]4] Lee, T. S., Krupa, R. A., Zhang, F., Hajimorad, M., Holtz, W. J., Prasad, N., … Keasling, J. D. (2011). BglBrick vectors and datasheets: A synthetic biology platform for gene expression. Journal of Biological Engineering, 5, 12. http://doi.org/10.1186/1754-1611-5-12