A glutamine-amidotransferase-like protein modulates FixT anti-kinase activity in Sinorhizobium meliloti.
BACKGROUND
Nitrogen fixation gene expression in Sinorhizobium meliloti, the alfalfa symbiont, depends on a cascade of regulation that involves both positive and negative control. On top of the cascade, the two-component regulatory system FixLJ is activated under the microoxic conditions of the nodule. In addition, activity of the FixLJ system is inhibited by a specific anti-kinase protein, FixT. The physiological significance of this negative regulation by FixT was so far unknown.


RESULTS
We have isolated by random Tn5 mutagenesis a S. meliloti mutant strain that escapes repression by FixT. Complementation test and DNA analysis revealed that inactivation of an asparagine synthetase-like gene was responsible for the phenotype of the mutant. This gene, that was named asnO, encodes a protein homologous to glutamine-dependent asparagine synthetases. The asnO gene did not appear to affect asparagine biosynthesis and may instead serve a regulatory function in S. meliloti. We provide evidence that asnO is active during symbiosis.


CONCLUSIONS
Isolation of the asnO mutant argues for the existence of a physiological regulation associated with fixT and makes it unlikely that fixT serves a mere homeostatic function in S. meliloti. Our data suggest that asnO might control activity of the FixT protein, in a way that remains to be elucidated. A proposed role for asnO might be to couple nitrogen fixation gene expression in S. meliloti to the nitrogen needs of the cells.
Bergès
H
H
Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, UMR215 CNRS-INRA, BP27, 31326 Castanet-Tolosan Cedex, France. hberges@toulouse.inra.fr
Checroun
C
C
Guiral
S
S
Garnerone
A M
AM
Boistard
P
P
Batut
J
J
D000878:Anthranilate Synthase; D001216:Asparagine / Q000502:physiology; D001217:Aspartate-Ammonia Ligase / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics / Q000502:physiology*; D019733:Carbon-Nitrogen Ligases with Glutamine as Amide-N-Donor / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology; D004251:DNA Transposable Elements / Q000235:genetics; D015870:Gene Expression; D019884:Nitrogenous Group Transferases / Q000502:physiology*; D010641:Phenotype; D010770:Phosphotransferases / Q000037:antagonists & inhibitors*; D016962:Sinorhizobium meliloti / Q000201:enzymology* / Q000235:genetics / Q000502:physiology
D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D001216:Asparagine; D019884:Nitrogenous Group Transferases; D010770:Phosphotransferases; D000878:Anthranilate Synthase; C020655:anthranilate synthase, glutamine amidotransferase subunit; D001217:Aspartate-Ammonia Ligase; D019733:Carbon-Nitrogen Ligases with Glutamine as Amide-N-Donor; C019742:asparagine synthetase (glutamine-hydrolyzing)