Biology Reference
In-Depth Information
2.2.2 Regulation of gene expression via protein
protein interaction
Next to the direct protein-DNA binding, the control of gene expression can
occur via the interaction of the haem-based sensor with the polymerase
machinery, thus indirectly influencing the transcription.
-
2.2.2.1 The nitrogen fixation gene expression regulator
In 1987, Ditta and coworkers published evidence that the regulation of
nitrogen fixation (
nif
) genes expression levels in
Sinorhizobium meliloti
(for-
merly known as
Rhizobium meliloti
) increased when environmental O
2
con-
centration dropped to microaerobic levels (
Ditta, Virts, Palomares, & Kim,
1987; Virts et al., 1988
). Not much later, a second paper appeared, reporting
the first characterization of FixL/FixJ, two proteins responsible for the O
2
dependence of
nif
genes expression (
David et al., 1988
). Soon after, FixL was
identified as an O
2
-binding hemoprotein, capable of sensing O
2
with its
haem group.
The unliganded form of FixL is catalytically active and transfers the
g
-phosphate of one ATP molecule to conserved aspartate residues of the
transcription factor FixJ (
Gilles-Gonzalez et al., 1991; Gilles-Gonzalez &
Gonzalez, 1993
). FixJ consists of a phosphorable N-terminal module and
a C-terminal transcription activator domain (
Batut, Santero, & Kustu,
1991
). The FixL-mediated phosphorylation derepresses the DNA-binding
domain of FixJ that becomes transcriptionally active (
Fig. 1.4
)(
Gouet
et al., 1999
).
FixL is a chimeric protein that consists of an optional membrane-anchoring
domain, a haem-binding PAS domain, and a histidine kinase (HK) domain.
The HK domain, in turn, is divided into an autophosphorylation subdomain,
where a histidine is the phospho-accepting residue (H box), and a catalytic
subdomain containing the ATP-binding site (
Lois, Ditta, & Helinski, 1993;
Monson, Weinstein, Ditta, & Helinski, 1992
). The activation of the kinase
Autophosphorylation
2ATP
2ADP
P
P
Transphosphorylation
Target genes
transcription
2FixL:2FixJ
2FixL:2FixJ
2FixL:2FixJ
Figure 1.4 FixL/FixJ activation scheme. The interaction between FixL and FixJ facilitates
the initial autophosphorylation of FixL, which transfers the g-phosphate group from an
ATP molecule with the production of one ADP. Subsequently, the transphosphorylation
occurs, with the transfer of the phosphate from FixL to FixJ. Phosphorylated FixJ is acti-
vated and stimulates the transcription of specific target genes.
