T. Palmer et al., INVOLVEMENT OF THE NARJ AND MOB GENE-PRODUCTS IN DISTINCT STEPS IN THE BIOSYNTHESIS OF THE MOLYBDOENZYME NITRATE REDUCTASE IN ESCHERICHIA-COLI, Molecular microbiology, 20(4), 1996, pp. 875-884
The Escherichia coil mob locus is required for synthesis of active mol
ybdenum cofactor, molybdopterin guanine dinucleotide. The mobB gene is
not essential for molybdenum cofactor biosynthesis because a deletion
of both mob genes can be fully complemented by just mobA. Inactive ni
trate reductase, purified from a mob strain, can be activated in vitro
by incubation with protein FA (the mobA gene product), GTP, MgCl2, an
d a further protein fraction, factor X. Factor X activity is present i
n strains that lack MobB, indicating that it is not an essential compo
nent of factor X, but overexpression of MobB increases the level of fa
ctor X, MobB, therefore, can participate in nitrate reductase activati
on. The narJ protein is not a component of mature nitrate reductase bu
t narJ mutants cannot express active nitrate reductase A. Extracts fro
m narJ strains are unable to support the in vitro activation of purifi
ed mob nitrate reductase: they lack factor X activity. Although the mo
b gene products are necessary for the biosynthesis of all E. coli moly
bdoenzymes as a result of their requirement for molybdopterin guanine
dinucleotide, NarJ action is specific for nitrate reductase A. The ina
ctive nitrate reductase A derivative in a narJ strain can be activated
in vitro following incubation with cell extracts containing the narJ
protein, NarJ acts to activate nitrate reductase after molybdenum cofa
ctor biosynthesis is complete.