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Role of the Sinorhizobium meliloti Global Regulator Hfq in Gene Regulation and Symbiosis

The RNA-binding protein Hfq is a global regulator which controls diverse cellular processes in bacteria. To begin understanding the role of Hfq in the Sinorhizobium meliloti--Medicago truncatula nitrogen-fixing symbiosis, we defined free-living and symbiotic phenotypes of an hfq mutant. Over 500 transcripts were differentially accumulated in the hfq mutant of S. meliloti Rm1021 when grown in a shaking culture. Consistent with transcriptome-wide changes, the hfq mutant displayed dramatic alterations in metabolism of nitrogen-containing compounds, even though its carbon source utilization profiles were nearly identical to the wild type. The hfq mutant had reduced motility and was impaired for growth at alkaline pH. A deletion of hfq resulted in a reduced symbiotic efficiency, although the mutant was still able to initiate nodule development and differentiate into bacteroids.