TABLE 1.

In vitro synthesis of the FeMo-co with purified components and the effect of addition of extracts of various Nif⁻ strains^a

Cell extract addedb	Genotype	Activity (nmol of ethylene formed/min)	Fold stimu-lation
None	None	0.46	1.0
DJ4248	ΔnifENX ΔvnfE	1.19	2.6
CA142	ΔnifDK ΔnifYENX::kan	1.21	2.6
DJ678	ΔnifDK ΔnifYENX::kan	0.95	2.1
DJ39	ΔnifNX34	0.85	1.8
UW NH4+-grown	nif repressed	0.80	1.7
DJ35	ΔnifE	12.75	27.7

^aThe reaction mixture contained 100 nmol of homocitrate, 10 nmol of molybdate, 200 μl of ATP-dithionite solution as defined in Materials and Methods, NifB-co (1 nmol of Fe), apodinitrogenase (12 μg of protein), NifNE (3 μg of protein), and dinitrogenase reductase (52 μg of protein) in a final volume of 550 μl. Twelve micrograms of dinitrogenase gave 15.5 nmol of ethylene formed/min when assayed with excess purified FeMo-co, 3 μg of NifNE gave 13.2 nmol of ethylene/min when assayed with extract of strain DJ35 (ΔnifE), NifB-co (1 nmol of Fe) gave 25.4 nmol of ethylene/min when assayed with an extract of strain UW45 (nifB mutant), and 52 μg of dinitrogenase reductase gave 82.4 nmol of ethylene/min when assayed with purified dinitrogenase, as described in Materials and Methods. 

^bWhere noted, 200 μl of extract (2.2 to 2.6 mg of protein) from the indicated strain was added.