Overview:
Our module aims at regulating our synthetic bacteria B. hercules. We first introduce a xylose inducible promoter, which can help us control the timing of BMP-2 expression. Our choice of xylose as an inducer stems from its induction efficiency, its little existence and low absorption rate in colon. (Yuasa et al., 1997) Besides the timing regulation, we introduce a cell growth inhibition device to prevent the overexpression of BMP-2. This device is achieved by a balance between a toxin and antitoxin pair ydcE and ydcD. By these two regulation systems, our B. hercules can produce more reliable and controllable performance.

Objective:

1. To regulate the expression of BMP-2 by a xylose inducible promoter. (Timing regulation)
2. To control the overexpression of BMP-2. (Dosage regulation)

Our Module in B. hercules:

1. The inducible promoter.

In the consideration of our B. hercules, one of our concerns is that our bacteria secrete BMP-2 before its binding with colon cancer cells. As a growth factor, while BMP-2 triggers the apoptosis of colon cancer cell, it can also stimulate the proliferation of normal epithelial cells in digestive tract. (Zhang et al., 2012) Thus, we intend to introduce a timing regulatory system into our B. hercules, which can be achieved by an inducible promoter.
Admittedly, there are various kinds of induction systems in Bacillus subtilis. However, to achieve the induction when our B. hercules is inside human colon, two conditions are required: first, the condition should not occur normally in vivo, but can be created in human colon; secondly, the inducer should not vitiate the healthy state of an individual. Besides, the efficiency of the induction is not required, but will be considerably helpful.
With those questions in mind, xylose is chosen as our inducer for B. hercules. It is the main building block for hemicellulose, which can only be found in plant. Largely absorbed in jejunum before reaching colon, xylose is not present in colon.1(Yuasa et al., 1997) Besides, the absorption rate of xylose in colon is low. Thus, well scheduled diet and medication can prevent the interaction of xylose and B. hercules before intestine and induction can therefore achieved by xylose delivered in enteric capsule or from anus. .
Besides its rare existence in the human colon, xylose is an efficient inducer as for PxylA promoter. When ligated with gene bgaB, 200-fold induction was achieved 30 minutes after the induction of xylose. (Kim, L. et al. 1996)


2. The Cell Growth Inhibition Device.

  Considering the unexpected proliferation of normal colon cells caused by over-dose BMP-2, a regulatory system is necessary for the dosage control of BMP-2 expression.2 (Zhang et al., 2012)
In order to build this system, we came up with a cell growth inhibition device to regulate. Understanding that toxin-antitoxin operons exist abundantly in bacteria, we intend to link the expression of BMP-2 with a toxin gene. However, the only existence of the toxin gene is not enough. Stabilization, to a certain extent, is necessary, so that our B. hercules will not die after a low level of BMP-2 expression.
In order to implement the ideas above, a toxin-antitoxin pair – ydcE and ydcD – is used. ydcE can encode an endoribonuclease – EndoA, which can cause cell growth inhibition. (For specific information, please click here) On the other hand, ydcD can encode YdcD, counteracting the effect of EndoA. (Pellegrini, O. et al. 2005) By linking ydcE immediately after BMP-2 gene, and put ydcD after pTms promoter, a relatively low efficient constitutive promoter, EndoA can be expressed simultaneously with the expression of BMP-2 under the control of xylose inducible promoter, and cell growth inhibition will not occur until the produced EndoA outweighs the effect of accumulated YdcD.