Cheng, H., C. A. Masiello, I. Del Valle, X. Gao, G. N. Bennett, and J. J. Silberg. 2018. Ratiometric gas reporting: A non-disruptive approach to monitor gene expression in soils. ACS Synthetic Biology 7:903-911.
Fluorescent proteins are ubiquitous tools that are used to monitor the dynamic functions of natural and synthetic genetic circuits. However, these visual reporters can only be used in transparent settings, a limitation that complicates non-disruptive measurements of gene expression within many matrices, such as soils and sediments. We describe a new ratiometric gas reporting method for non-disruptively monitoring gene expression within hard-to-image environmental matrices. With this approach, C2H4 is continuously synthesized by ethylene forming enzyme to provide information on viable cell number, and CH3Br is conditionally synthesized by placing a methyl halide transferase gene under the control of a conditional promoter. We show that ratiometric gas reporting enables the creation of Escherichia coli biosensors that report on acylhomoserine lactone (AHL) autoinducers used for quorum sensing by gram-negative bacteria. Using these biosensors, we find that an agricultural soil decreases the bioavailable concentration of a long-chain AHL up to 100-fold. We also demonstrate that these biosensors can be used in soil to non-disruptively monitor AHLs synthesized by Rhizobium leguminosarum and degraded by Bacillus thuringiensis. Finally, we show that this new reporting approach can be used in Shewanella oneidensis, a bacterium that lives in sediments.
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