AICAR, a purine-related metabolite, was recently shown to inhibit respiratory and nifA gene expression in Sinorhizobium meliloti. Here, we demonstrate that AICAR has essentially no or little effect in a wild-type S. meliloti strain and inhibits respiratory and nitrogen fixation gene expression only in specific mutant backgrounds. We have analyzed in detail a mutant in which addition of AICAR inhibited fixK, fixN, fixT, and nifA expression. The corresponding gene, fixM, is located just downstream of fixK 1 on pSymA megaplasmid and encodes a flavoprotein oxidore-ductase. 5′AMP, a structural analogue of AICAR, mimicked AICAR effect as well as the nucleoside precursors AICAriboside and adenosine. The mode of action of AICAR and 5′AMP in vivo was investigated. We demonstrate that AICAR does not affect FixK transcriptional activity and instead regulates fixK and nifA gene expression. We hypothesize that AICAR and 5′AMP may modulate, possibly indirectly, the activity of the FixLJ two-component regulatory system. The possible physiological roles of AICAR, 5′AMP, and fixM in the context of symbiosis are discussed.