Introduction

The interaction between rhizobia and their host plants culminates in the formation of specialized organs called nodules in which differentiated bacteria fix nitrogen to the benefit of the host plant. The main focus of our group is the identification of the physiological signals that control the expression of bacterial genes important for nitrogen fixation. Symbiotic expression of most of the nitrogen fixation genes identified so far in Sinorhizobium meliloti is under the control of the two component regulatory system fixLJ that responds to the microaerobic environment in the nodule (David et al. 1989). Soberon et al. (2001) reported that "AICAR" a purine intermediate down-regulates fixN expression in S. meliloti. Recently, we showed that a 151 amino acid protein of previously unknown function (Batut et al. 1989) modulates inhibition by "AICAR".

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