A purine-related metabolite negatively regulates fixNOQP expression in Sinorhizobium meliloti by modulation of fixK expression

5-aminoimidazole-4-carboxamide nucleotide (AICAR) is a negative effector of cytochrome terminal oxidase cbb3 production in Rhizobium etli, In this wor k, the effect of AICAriboside (AICAr), the precursor of AICAR on the expres sion of the Sinorhizobium meliloti fixNOQP operon encoding the symbiotic te rminal oxidase cbb3, was analyzed. AICAr reduced the microaerobic induction levels of fixN-lacZ and fixT-lacZ gene fusions 18- and sevenfold:respectiv ely, and both genes were activated by the transcriptional activator FixK, A fucK-lacZ fusion presented 14-fold-reduced induction levels in microaerobi c cell cultures in the presence of AICAr, AICAr also reduced three-fold the microaerobic expression levels of the nifA-lacZ fusion, whose expression a s well as that of fixK is controlled by the two-component system FixL-FixJ. In contrast, AICAr had no effect on the expression levels of a hemA-lacZ f usion. These data suggest that AICAr prevents fixNOQP induction by the inhi bition of fixK transcription.