The biological circuit

Signal production by mine cells (sender cells)

Mine colonies constitutively express the "mine-cell reporter” hydrolase nagZ [1] that reacts with the specific substrate in the multi-substrate solution to produce a chromogenic product. The colony turns blue, indicating that the colony is a mine and hence the game ends. At the same time, the mine colony is a sender module of OHHL. LuxI is expressed constitutively and synthesizes the OHHL signaling molecule. This means that the mine continuously produces OHHL, that gradually diffuses radially out to neighboring colonies. The signaling molecule OHHL is processed by the receiver cells in the agar mine grid.

Figure 3: [OHHL] dependent information processing

Information processing in non-mine cells (receiver cells)
So what exactly happens to the diffused OHHL?

1. Receiver cells respond to low OHHL levels: If one mine cell is close to a non-mine colony and by OHHL diffusion takes place to reach the non-mine colony. Due to a high-pass filter using the LuxR system in the non-mine colony, low concentrations of OHHL are detected and activate a reporter. Addition of the multi-substrate to this colony produces a salmon color, which indicates one mine is adjacent to the colony just played.

The wild type promoter BBa_R0062 PLuxR is used for the detection of low OHHL concentrations and therefore become activated when one mine cell is in the vicinity of the receiver cell. Then the wild type promoter induces the expression of gusA, a hydrolase able to convert a specific substrate to a visible red output (see data page).

2. Receiver cells respond to high OHHL levels: If there are two or more mine colonies adjacent to a non-mine colony, a higher concentration of diffused OHHL will be processed by the receiver colony. The LuxR system works as a high-pass filter again and induces the expression of another reporter to show that two or more mines surround the selected colony. Addition of the multi-substrate to this colony produces a magenta color that indicates that more than one mine is adjacent to this colony.

The mutated promoter PluxR BBa_K1216007 will be used in our final set-up to only respond to high concentrations of OHHL and therefore be activated, when two sender cells are surrounding the receiver cell, and expressing the aes AES hydrolase responsible for the colorimetric conversion of the specific substrate to a visible purple output.

3. Receiver cells signal with no mines in their vicinity. If there is no mine adjacent to a non-mine colony, no OHHL is processed. Only the constitutive hydrolase phoA is expressed, which on addition of the multi-substrate will give a yellow color. This indicates that there are no mines adjacent to this colony and the game can be continued.

Flagging
Placing a flag on a mine prevents the mine to detonate. We have implemented this feature of the computer game in Colisweeper. If the player is sure about a colony to be a mine, a second single substrate can be added on to this colony. All colonies express the native hydrolase lacZ reporter for flagging, a green colored product indicates a flagged colony. But remember, flagging does not reveal any information about mines in neighboring cells.

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