Isolation and characterization of adenylate kinase (adk) mutations in Salmonella typhimurium which block the ability of glycine betaine to function as an osmoprotectant

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Abstract
Mutants of Salmonella typhimurium that were not protected by glycine betaine (GB) but could still use proline as an osmoprotectant in media of high osmolality were isolated. The mutations responsible for this phenotype proved to be alleles of the adenylate kinase (adk) gene, as shown by genetic mapping, sequencing of the cloned mutant alleles, complementation with the Escherichia coli adk gene, and assay of Adk enzyme activity in crude extracts. One of the mutations was in the untranslated leader of the adk mRNA, a second was in the putative Shine-Dalgarno sequence, and a third was in the coding region of the gene. The loss of osmoprotection by GB was shown to be due to the fact that the accumulation of this solute actually resulted in a severe inhibition of growth in the adk mutants. The addition of GB in the presence of 0.5 M NaCl resulted in a rapid decline in the ATP pool and a dramatic increase in the AMP pool in the mutants. Proline, which is not toxic to the adk mutants, did not have any significant effects on the cellular levels of ATP and AMP. The mutants exhibited two different phenotypes with respect to the utilization of other osmoprotectants: they were also inhibited by propiothiobetaine, L-carnitine, and gamma-butyrobetaine, but they were stimulated normally in media of high osmolality by proline, choline-O-sulfate, and stachydrine.