TABLE 4.

Effect of ammonium ions, oxygen, and Fe on the activity of the H. seropedicae NifA proteins expressed in A. brasilense^a

Treatment	β-Galactosidase activity (Miller units)
	FP10(pEMS135) (nifH::lacZ)	FP10(pEMS136) (nifH::lacZ)
1. −N/−O	152	148
2. −N/+O	7	15
3. +N/−O	12	207
4. +N/+O	2	6
5. −Fe	8	2
6. −Fe/+EDTA	6	2
7. +EDTA/+Fe	124	132

^aAzospirillum brasilense FP10 (nifA) carrying either pEMS135 (native NifA) or pEMS136 (N-terminally truncated NifA) was grown in NFbHP plus ammonium ions (20 mM) in air at 30°C overnight. The cells were then centrifuged and resuspended in NFbHP containing glutamate (0.1 mM) and derepressed for NifA activity under the following treatments: 1, absence of NH₄Cl and 0.5% oxygen; 2, absence of NH₄Cl and air; 3, NH₄Cl (20 mM) and 0.5% oxygen; 4, NH₄Cl (20 mM) and air; 5, treatment 1 but without added Fe; 6, treatment 5 plus EDTA (0.2 mM); and 7, treatment 6 plus FeSO₄ · 7H₂O (20 μg/ml). β-Galactosidase activities were measured after 4 h at 30°C.