Experimental Studies of Signal Noise in Gene Regulation in the Inducible Antibiotic Resistance Pathway of Escherichia coli

Katherine E. Frato

Since gene regulation is governed by nonlinear equations, we hypothesized that thermodynamic noise in the environmental chemicals that interact with proteins and DNA regulatory regions may enhance signal detection. This study used the mar pathway in E. coli as a model of gene regulation. The mar pathway detects environmental signals such as sodium salicylate and induces a multidrug antibiotic resistance phenotype. In order to measure the response of a gene regulatory pathway to environmental noise, E. coli strains were constructed with the green fluorescent protein (GFP) under the control of two separate promoters from different steps of the mar signal cascade. The plasmid-based constructs were transferred to the E. coli chromosome using the phase λInCh2 to ensure only a single copy of the sequence was present. Cells carrying the mar construct were induced with various concentrations of sodium salicylate and fluorescence intensity of individual cells was measured by fluorescence microscopy. The mean fluorescence intensity showed a minimum at 4mM concentrations of sodium salicylate, but the phenotypic noise showed no clear trend over sodium salicylate concentrations. The minimum in mean fluorescence intensity may be a result of stochastic anti-resonance or additional complexity in the biochemical pathway. Further research is required to investigate the possible causes of the minimum in the mean fluorescence intensity and to identify trends in phenotypic noise over inducer concentrations.