Additional file 3.

Figure S1. Intra and extracellular autoinducer as a function of exogeneous autoinducer concentration. Response curves to autoinducer induction for lux01 (A, C and E) and lux02 (B, D and E) operons. Total autoinducer concentration <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M17">View MathML</a> in the external volume and in the cells (A and B), intracellular concentration cA(C and D), and extracellular concentration <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M18">View MathML</a> (E and F), as a function of the exogenous autoinducer concentration, <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M19">View MathML</a>, in the deterministic model. All graphs represent the steady-state response for increasing (blue curve) and decreasing (red curve) autoinducer concentrations. The exogeneous autoinducer concentration <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M20">View MathML</a> controls the autoinducer concentration in the medium by means of an influx and an efflux (see main text). Upon activation of the operon, LuxR is produced at high levels, thus sequestering autoinducer molecules inside the cells. The bound form of autoinducer cannot diffuse out of the cell and is therefore not subjected to the influx and efflux. This explains why the total concentration of autoinducer in the system, <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M21">View MathML</a> is slightly larger than <a onClick="popup('http://www.biomedcentral.com/1752-0509/7/6/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1752-0509/7/6/mathml/M22">View MathML</a>, when the operon is activated. For the same reason, the free form of autoinducer, both in the cell and in the medium, is slightly smaller.

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Weber and Buceta BMC Systems Biology 2013 7:6   doi:10.1186/1752-0509-7-6