Biological Logic Gates

As I have mentioned before, logic gates are not restricted to the textbooks of electronics, they are also exciting stuff for system biologists!

This group has constructed a bacterial 'AND' logic gate, and the Boolean logic behind its operation should be acceptable to any readers who has done Physics or Pascal Programming. The idea is to use 2 chemical molecules ('C12' and 'C4') as input signals and this leads to the biosynthesis of a molecule called phenazine. Phenazine provides an electron source and creates an electric current (in milliamperes). Thus the observable output is the generation of the electric current. This idea is indeed inspired by the biosynthetic route that takes place in the bacteria (see Figure 1).


Figure 1. The inspiration.


Figure 2. Some findings from the investigation.

We can all appreciate the outcome of the experiment - only when both input chemical signals are TRUE (when both C12 and C4 are present) will we have a TRUE output - electric current is generated. One intriguing aspect I will like to draw your attention to is the observation that when C4 is present alone in the medium, it generates a somewhat larger current than when C12 is present. (Figure 2) In theory, since both of these scenarios will contribute to a FALSE output, their result in the current reading should be similar. The explanation offered by the team is that as C4 comes later in the synthetic sequence, so its sole presence somehow contributes to a 'fake' signal to the system that the output should be TRUE. Obviously, since C12 is absent, it will not be possible to be assigned a TRUE value by Boolean Logic - therefore it is still FALSE. The conclusion: very impressive idea!

Reference:

Bacteria-based AND logic gate: a decision-making and self-powered biosensor Zhongjian Li, Miriam A. Rosenbaum, Arvind Venkataraman, Tsz Kin Tam, Evgeny Katz and Largus T. Angenent
Chem. Commun., 2011, Advance Article
DOI: 10.1039/C0CC05037G