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<p>Abstract</p> <p>Background</p> <p>Nitrogen fixation gene expression in <it>Sinorhizobium meliloti</it>, 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.</p> <p>Results</p> <p>We have isolated by random Tn5 mutagenesis a <it>S. meliloti</it> 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 <it>asnO</it>, encodes a protein homologous to glutamine-dependent asparagine synthetases. The <it>asnO</it> gene did not appear to affect asparagine biosynthesis and may instead serve a regulatory function in <it>S. meliloti</it>. We provide evidence that <it>asnO</it> is active during symbiosis .</p> <p>Conclusions</p> <p>Isolation of the <it>asnO</it> mutant argues for the existence of a physiological regulation associated with <it>fixT</it> and makes it unlikely that <it>fixT</it> serves a mere homeostatic function in <it>S. meliloti</it>. Our data suggest that <it>asnO</it> might control activity of the FixT protein, in a way that remains to be elucidated. A proposed role for <it>asnO</it> might be to couple nitrogen fixation gene expression in <it>S. meliloti</it> to the nitrogen needs of the cells.</p>