| Summary: | Bulk soil is a stressful environment low in readily available nutrients and characterized by a low oxygen tension. Additionally, many soils undergo conditions of drought. Strategies that common soil bacteria take to cope with adverse conditions may be crucial to the ability of soil microorganisms to persist in soil. Therefore, studying genetic loci inducible by stress conditions commonly applied to soil microorganisms may shed light on the strategies used, and a thorough understanding of molecular responses to these stresses is critical. This dissertation explores in detail the responses of S. meliloti 1021 to experimentally induced salinity and desiccation. We have identified a number of factors central to S. meliloti 1021's ability to survive desiccation, including temperature, stationary growth phase, chloride and sulfate availability. We were also successful in identifying two genetic loci that are important in the adaptation of S. meliloti 1021 to increased levels of NaCl. One such locus ( asnO) was shown to contain a hypothetical glutamine-dependent asparagin synthetase. We showed that asnO::tn5LuxAB (a transcriptional fusion with luciferase on a Tn5 derivative) was induced by chloride and osmotic stress, and during the stationary phase. The main function of AsnO is not asparagine synthesis since addition of aminoacids to cells at elevated NaCl concentrations did not complement growth. However, AsnO is not involved in NaCl mediated survival during desiccation. We also tested a second locus for its involvement in NaCl response and desiccation. Open reading frame smb20482, contains a putative dAla-dala ligase domain and is induced by osmotic stress. Disruption of this locus caused NaCl mediated desiccation sensitivity. Penicillin sensitivity indicates a function for this gene in crosslinking of peptidoglycan. Interestingly, homologs of this gene are only found among the proteobacteria. In conclusion, (1) chloride availability enhances survival during desiccation of S. meliloti 1021, (2) we identified two NaCl responsive loci also involved in adaptation to NaCl and, (3) one locus (smb20482) is also involved in survival during desiccation. Thus, our strategy to identify genetic loci involved in the survival during desiccation has been successful.
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