Bergès H, Checroun C, Guiral S, Garnerone AM, Boistard P, Batut J. A glutamine-amidotransferase-like protein modulates FixT anti-kinase activity in Sinorhizobium meliloti.
BMC Microbiol 2001;
1:6. [PMID:
11389771 PMCID:
PMC32199 DOI:
10.1186/1471-2180-1-6]
[Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2001] [Accepted: 05/22/2001] [Indexed: 11/10/2022] Open
Abstract
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.
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