Genetic regulation of iron responsive genes in sinorhizobium meliloti

Iron is an essential nutrient for most bacteria. It is a crucial metal of many metalloenzymes and functions m important biological systems mamly as the cofactor of redox enzymes Bacteria must acquire iron from the environment where the metal is mainly found in the fem e iron state, which is very insoluble. In addition, they must maintain iron homeostasis. One mechanism used by bacteria for the acquisition of iron is the production of siderophores, which are low molecular weight chelators with affinity and specificity for ferroc iron and which are formed and secreted under iron deplete conditions.\ud \ud The regulation of iron was studied in Sinorhizobium mehloti, which is a free-living Gram-negative bacteria found in soil and also as an endosymbiont o f Medicago sativa (alfalfa). A homologue of the ferric uptake protein (Fur), which regulates the uptake of iron in most Gram-negative bacteria, was identified and characterised However, the results suggest that in S mehloti, Fur does not function as an iron response regulator but actually regulates manganese uptake. Another protein, the homologue of the transcriptional iron regulator RirA in Rhizobium legummosarum was identified and characterised in S mehloti as the new general regulator of iron responsive genes. Results showed that RirA, under iron replete conditions, downregulates the rhizobactin 1021 siderophore biosynthesis genes and also the gene encoding the outer membrane receptor of the chelator. In addition, RirA was found to downregulate and upregulate respectively smc02726 and d ppAl, genes involved in haem acquisition, indicating that the regulator can function both as an activator and a repressor. Also, results showed the upregulation o f rhbG, a putative rhizobactin 1021 siderophore gene by luteohn, a flavonoid produced by alfalfa, under iron deplete and also under iron replete conditions