Furthermore, the roles of the reductases encoded by napA, nirK, norC and nosZ in nitrite, nitric oxide, N2O production and N2O reduction, respectively, were demonstrated. Thus, our results contribute to the investigation of the unexplored genetic basis for denitrification in the alfalfa endosymbiont E. meliloti. This knowledge will be instrumental in the

development of agricultural strategies and management practices for mitigating the release of N2O from legume crops. Selleckchem AZD3965 Methods Bacterial strains and growth conditions The bacterial strains used in this study are listed in Table 1. E. meliloti strains were routinely grown aerobically at 30°C in tryptone yeast (TY) complete mTOR inhibitor medium [43]. These cultures were then used as the inocula for subsequent incubation experiments, which were performed in minimal medium (MM) [44] or in MM medium supplemented with 10 mM KNO3 Tipifarnib (MMN); the cells were subjected to two experimental oxygen-limiting conditions. In the first set of experiments, 17-ml serum tubes or 500-ml flasks containing 5 or 200 ml medium, respectively, were sealed with rubber septa, and the headspace atmospheres were replaced with a gas mixture (2% oxygen, 98% argon) at the starting point of the incubation. In the second experiment, the cells were incubated in completely filled 200-ml bottles or 17-ml tubes without added oxygen; these conditions are referred to throughout

the manuscript as “anoxic conditions”. Antibiotics were added to the cultures at the following concentrations (μg · ml-1): streptomycin, 200; and kanamycin, 200. Headspace O2 measurements After inoculation at an OD600 of 0.2, 1 ml of each culture was placed in a 3-ml thermostatted and magnetically stirred reaction chamber with an O2 electrode (Hansatech, Norkfolk, England). The headspace atmosphere in the chamber was replaced with a gas mixture (2% oxygen, 98% argon) at the starting point of the incubation. The kinetics of oxygen depletion in the chamber were monitored.

Determination of nitrate reductase Ponatinib and nitrite reductase activity E. meliloti cells were incubated (initial OD600 of approximately 0.15-0.2) under 2% initial oxygen or under anoxic conditions for 18 h in MMN medium. The cells were harvested by centrifugation at 8000 g for 10 min at 4°C, washed with 50 mM Tris/HCl buffer (pH 7.5) until no nitrite was detected and then resuspended in 0.5 ml of the same buffer. The methyl viologen-dependent nitrate reductase (MV+-NR) activity was analysed essentially as described by Delgado and colleagues (2003) [32]. To determine the methyl viologen-dependent nitrite reductase (MV+-Nir) activity, the reaction mixture contained 50 mM Tris/HCl buffer (pH 7.5), 200 μM NaNO2, 400 μM methyl viologen (MV) and 100 μl of cell suspension (0.02–0.04 mg of protein). The reaction was started by the addition of 50 μl of freshly prepared sodium dithionite solution (30 mg · ml-1 in 300 mM NaHCO3).