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As known, bacteria are able to respond to multiple stimuli through a collection of chemoreceptors which can sense different sensory modalities and produce chemotactic responses as a cooperative effort. Chemotaxis process is regulated by a high conserved transduction-signal system, which can be modeled and analyzed as a interconnected network of protein interactions. This system represent a great example of how cellular circuit evolve to generate functional outputs. In chemotaxis, whereas signal-transduction system is completely regulated at genetically level, flagella regulation depends on genetic expression and protein function.

In this second circuit, our main aim is concentrate a significant population of bacteria around some vegetable polysaccharidic surface. This phenomenon was renamed as BACTERIAL CROWDING. Therefor, it is necessary that the process of interaction with the surface by a few number of bacteria triggered the production and excretion of chemicals that, acting as chemoattractants, generate a chemical-diffusing gradient which could induce chemotactic process of close bacterias. This bacteria will redirect their random movements to the same plant surface and the concentration of the cell population will raise up in this region, thanks to amplification process.

To that effect, PrhI-dependent promoter PprhJ will activate specifically chemoattractant production (aspartate, glutamate or salicylate), in the same way that FecI-dependent promoter PfecA. And we compare the two circuits with Fec circuit, which is common among E. coli. The four possible circuits are represented below.

We will also make use of Tar, an innate E. coli chemotactic receptor, which is divided in two transmembrane sequences, constituting a periplasmic domain and a cytoplasmic domain. Periplasmic domain have a dimeric structure with four helix, that binds to aspartate and glutamate asymmetrically. Conformational change generated by such union is transmitted to cytoplasmic region, highly conserved, responsible of the activation of the flagellar motor. If we use salicylate, the system will be composed by two bacteria: E. coli (detection population) and P. putida (chemotactic population), because the fist one is not chemotactic to salicylate.

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