Results and Discussion

Sequencing the rhizobial DNA flanking the minitransposon insertions revealed that genes required for cytochrome biosynthesis, DNA modification, lipid metabolism, membrane transport, and regulation were transcriptionally activated by low pH. Other insertions disrupted membrane proteins of unknown function.

To detect low pH-inducible proteins possibly essential for growth under acidic conditions, we compared protein profiles of cells exposed to a persistent or transient acid-stress. We found approximately 50 proteins that are pH-regulated; sixteen of which yielded N-terminal sequences after Edman degradation. Transient acid exposure down-regulated proteins involved in nitrogen metabolism and up-regulated a hypothetical protein. Continuing acid exposure down-regulated proteins required for amino-acid transport, proteolysis, lipid metabolism, and unknown proteins. Persistent acid-stress up-regulated chaperone proteins and others involved in proteolysis and carbon metabolism. The non-overlap of the proteins identified by the two methods is probably due to: loss of proteins of IEP>7 or <4, loss of membrane proteins or greater sensitivity of the fusion technique for proteins of low copy number. Overall, the total number of proteins involved in pH-response and acid-tolerance may be as high as 100.

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