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kinases sense external signals, by which the autokinase activities of the sensor
kinases are regulated. Once the sensor kinase is activated to take place its
autophosphorylation, the phosphoryl group added in the sensor kinase trans-
fers to the cognate response regulator to form a phosphorylated response
regulator, which results in the regulation of the biological function of
response regulator.
FixL is a sensor kinase in the FixL/FixJ two-component system that reg-
ulates the expression of nitrogen fixation genes in response to O 2 concen-
tration in nodule bacteria ( Bauer, Elsen, &Bird, 1999; Fischer, 1994; Sciotti,
Chanfon, Hennecke, & Fischer, 2003 ). Bradirhizobium japonicum and
Sinorhizobium meliloti are symbionts that form nodule upon infecting to soy-
bean and alfalfa roots, respectively, FixL proteins from which have been
studied extensively. In nodules, a series of nitrogen fixation genes are
expressed as O 2 concentration decrease < 50 m M( Gilles-Gonzalez, 2001;
Soup`ne, Foussard, Boistard, Truchet, & Batut, 1995 ). As nitrogenase, a
key enzyme for nitrogen fixation to form ammonia from N 2 , is sensitive
to O 2 , its O 2 -dependent expression is strictly regulated to prevent nitroge-
nase from inactivation by O 2 .In S. meliloti , the FixL/FixJ system regulates
the expression of fixK and nifA genes, while only fixK 2 gene is regulated by
the FixL/FixJ system in B. japonicum ( David et al., 1998; Fischer, 1994 ). As
FixK, NifA, and FixK 2 are transcriptional regulators, the regulatory cascades
for gene expression proceed by a cue triggered with the FixL/FixJ two-
component system.
FixLconsists of the sensor andkinasedomains,which is autophosphorylated
with ATP, and then phosphoryl group transfer takes place between FixL and
FixJ ( Gilles-Gonz´lez, Gonz´lez, &Perutz, 1995; Gilles-Gonzalez et al., 1994 ).
Unphosphorylated FixL preferably exists in the formof quaternary complex of
(FixL) 2 (FixJ) 2 ( Miyatake et al., 1999a,b ). Phosphorylation of FixL/FixJ takes
place in the (FixL) 2 (FixJ) 2 complex, inwhichATPbound inone subunit is used
to phosphorylate the conserved His (His285 in Sm FixL) in the other subunit
( Gilles-Gonzalez & Gonzalez, 1993; Monson, Ditta, & Helinski, 1995;
Nakamura, Kumita, Imai, Iizuka, & Shiro, 2004; Reyrat, David, Batut, &
Boistard, 1994; Tuckerman,Gonzalez,&Gilles-Gonzalez, 2001 ).Whenphos-
phoryl group transfer has proceeded from the phosphorylated FixL to FixJ,
phosphorylated FixJ is released from the complex to form a homodimer of
phosphorylated FixJ. Upon phosphorylation, FixJ forms a homodimer that is
the active form for the transcriptional regulator with specific DNA-binding
activity ( Agron, Ditta, & Helinski, 1993; Da Re et al., 1999; Reyrat, David,
Blonski, Boistard, & Batut, 1993 ). O 2 inhibits the phosphorylation reaction
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