Expression of a nitrogen regulated lux gene fusion in Pseudomonas fluorescens DF57 studied in pure culture and in soil

The biological availability of assimilable nitrogen in soil was assessed by introducing a Pseudomonas fluorescens DF57 reporter strain, carrying a Tn5::luxAB insertion in a nitrogen-starvation-inducible gene, displaying no homology with any known genes. Initial pure culture experiments demonstrated that bioluminescence was induced in response to limitation for ammonium or other nitrogen sources that could be easily assimilated by the P. fluorescens strain. Conditions conducive to the expression of bioluminescence existed at ammonium concentrations below 10–90 μM. Upon introduction into sterile or natural soil microcosms a nitrogen-starvation signal was not observed, demonstrating that the cells were not encountering a limited supply of nitrogen sources. However, amendment of the soil with carbon+phosphorus sources led to conditions where a specific nitrogen-starvation signal was observed. Attempts to repress the reporter system by subsequent addition of ammonium were unsuccessful in the soil, but not in pure culture experiments. A possible explanation for this result is that the luciferase enzyme is stable in cells inhabiting the soil environment, imposing limitations to the use of luxAB as a real-time monitoring system.