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 assess ed by introducing a Pseudomonas fluorescens DF57 reporter strain, carr ying a Tn5::luxAB insertion in a nitrogen-starvation-inducible gene, d isplaying no homology with any known genes. Initial pure culture exper iments demonstrated that bioluminescence was induced in response to li mitation for ammonium or other nitrogen sources that could be easily a ssimilated by the P. fluorescens strain. Conditions conducive to the e xpression of bioluminescence existed at ammonium concentrations below 10-90 mu M. 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, Howe ver, amendment of the soil with carbon+phosphorus sources led to condi tions where a specific nitrogen-starvation signal was observed, Attemp ts to repress the reporter system by subsequent addition of ammonium w ere unsuccessful in the soil, but not in pure culture experiments. A p ossible explanation for this result is that the luciferase enzyme is s table in cells inhabiting the soil environment, imposing limitations t o the use of luxAB as a real-time monitoring system. (C) 1998 Federati on of European Microbiological Societies. Published by Elsevier Scienc e B.V.