Knowledge of the stress-activated genetic circuits in S. meliloti is required to understand how acid-tolerant strains can survive in acidic soils to form an effective symbiosis. To identify global changes in S. meliloti in response to pH stress we have used two approaches. With the first approach, minitransposon-induced transcriptional fusions to a promoterless gusA (Reeve et al. 1998, 1999) were used to identify pH-responsive sinorhizobial genes. With the second approach, the pH-responsive protein complement was identified using proteomic analysis (Guerreiro et al. 1999; Natera et al. 2000; Chen et al. 2000).
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