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Background: The bacterial Hfq protein is able to interact with diverse RNA molecules, including regulatory small non- coding RNAs (sRNAs), and thus it is recognized as a global post-transcriptional regulator of gene expression. Loss of Hfq has an extensive impact in bacterial physiology which in several animal pathogens influences virulence. Sinorhizobium meliloti is a model soil bacterium known for its ability to establish a beneficial nitrogen-fixing intracellular symbiosis with alfalfa. Despite the predicted general involvement of Hfq in the establishment of successful bacteria-eukaryote interactions, its function in S. meliloti has remained unexplored. Results:Two independentS. melilotimutants, 201 1-3.4and lO2lΔhfq,were obtained bydisruption and deletion ofthe hfq gene in the wild-type strains 2011 and 1021, respectively, both exhibiting similar growth defects as free-living bacteria. Transcriptomic profiling of 1 O2lAhfq revealed a general down-regulation of genes of sugar transporters and some enzymes of the central carbon metabolism, whereas transcripts specifying the uptake and metabolism of nitrogen sources (mainly amino acids) were more abundant than in the wild-type strain. Proteomic analysis of the 2011-3.4 mutant independently confirmed these observations. Symbiotic tests showed that lack of Hfq led to a delayed nodulation, severely compromised bacterial competitiveness on alfalfa roots and impaired normal plant growth. Furthermore, a large proportion of nodules (55%-64%) elicited by the 102 lAhfq mutant were non-fixing, with scarce content in bacteroids and signs of premature senescence of endosymbiotic bacteria. RT-PCR experiments on RNA from bacteria grown under aerobic and microoxic conditions revealed that Hfq contributes to regulation of nifA and fixKl/K2, the genes controlling nitrogen fixation, although the Hfq-mediated regulation of fixKis only aerobiosis dependent. Finally, we found that some of the recently identified 5. meliloti sRNAs co-inmunoprecipitate with a FLAG- epitope tagged Hfq protein. Conclusions: Our results support that the 5. meliloti RNA chaperone Hfq contributes to the control of central metabolic pathways in free-living bacteria and influences rhizospheric competence, survival of the microsymbiont within the nodule cells and nitrogen fixation during the symbiotic interaction with its legume host alfalfa. The identified 5. meliloti Hfq-binding sRNAs are predicted to participate in the Hfq regulatory network. [ABSTRACT FROM AUTHOR]