RM/lac Hybrid Promoter Assay

1. Introduction

In order to achieve the natural plants' temporal pattern for producing plant hormones in E. coli, we introduced an incoherent feed forward loop to our circuit, including a new hybrid promoter part ([http://parts.igem.org/Part:BBa_K1139150 BBa_K1139150]). Plants produce their hormones transiently rather than steadily (Takei et al., 2001) (Fig. 3-7-1). Moreover, continuous overexpression of hormones is harmful to plants (Thiman, 1937). Thus, we thought that it should be important to achieve this transient temporal pattern for producing plant hormones in E. coli, too. By applying the incoherent feed forward loop, the transient pulse of gene expression can be generated (Basu et al., 2003; Mangan et al., 2006).

Fig. 3-7-1. Temporal pattern for producing plant hormones in plants

Our designed system with an incoherent feed forward loop is shown in Fig. 3-7-2. We newly developed the RM/lac hybrid promoter, which is activated by CI and repressed by LacI (Fig. 3-7-3). We planned to ligate a hormone synthase part downstream of this hybrid promoter. Our mathematical model (Fig. 3-7-4) shows the pulse wave which the temporal pattern for plant hormone production should achieve (details about this model can be found here). While the RM/lac hybrid promoter activation by CI is a single-step reaction, the repression by LacI is a two-step reaction. Thus, the activation of the RM/lac hybrid promoter is faster than the repression. This time lag between the activation and the repression is important for generating a temporal pattern of plant hormone production.

Fig. 3-7-2. Our designed system with an incoherent feed forward loop

Fig. 3-7-3. Our newly designed hybrid promoter

Fig. 3-7-4. Our mathematical model for temporal pattern generation

As a first step to achieve this system with an incoherent feed forward loop, we constructed a circuit shown in Fig. 3-7-5 to confirm that our new RM/lac hybrid promoter actually works. We set GFP as a reporter of the RM/lac hybrid promoter and introduced the part into E. coli . We verified that our RM/lac hybrid promoter was activated by CI and repressed by LacI with the following assay.

Fig. 3-7-5. The circuit we constructed including the new part

2. Materials and Methods

Fig. 3-7-6. Constructions for our assay

2-1. Construction

-pSB6A1-Ptet-GFP (N99)…positive control

-pSB6A1-promoterless-GFP (N99)…negative control

-pSB6A1-PRM/lac-GFP (N99)…sample with CI*

-pSB6A1--PRM/lac-GFP (JM2.300)…sample without CI

* This N99 strain expresses CI from its genome constitutively.

2-2. Assay protocol

3. Results

Fig. 3-7-7 shows the fluorescence intensity detected by flow cytometer. Fig. 3-7-8 is the extracted data which shows the comparison of N99 (IPTG+, with constitutive CI) and JM2.300 (IPTG+, without CI).

Fig. 3-7-7. Fluorescence intensity detected by flow cytometer

Fig. 3-7-8. Comparison of N99 and JM2.300

4. Discussion

N99 cells (CI+) showed higher fluorescence intensity than that of JM2.300 cells (CI-). From this result, we assume that our RM/lac hybrid promoter was actually activated by CI. In addition, N99 (IPTG-) showed lower fluorescence than that of N99 (IPTG+). From this result, we can assume that our RM/lac hybrid promoter was repressed by LacI derived from the E. coli genome.

5. References

1. Kentaro Takei, Hitoshi Sakakibara, Mitsutaka Taniguchi and Tatsuo Sugiyama. (2001) Nitrogen-Dependent Accumulation of Cytokinins in Root and the Translocation to Leaf: Implication of Cytokinin Species that Induces Gene Expression of Maize Response Regulator. Plant Cell Physiol. 42, 85–93
2. Kenneth V. Thiman. (1937) On The Nature of Inhibitions Caused by Auxin. American Journal of Botany. Vol. 24, No. 7, 407-412
3. Basu S, Mehreja R, Thiberge S, Chen MT, Weiss R. (2003) Spatiotemporal control of gene expression with pulse-generating networks. Proc. Natl. Acad. Sci. USA, 101, 6355-6360.
4. S. Mangan, S. Itzkovitz, A. Zaslaver, U. Alon. (2006) The Incoherent Feed-forward Loop Accelerates the Response-time of the gal System of Escherichia coli . J. Mol. Biol. 356, 1073-1081