Salinity-Dependent Switching of Osmolyte Strategies in a Moderately Halophilic Bacterium: Glutamate Induces Proline Biosynthesis in Halobacillus halophilus

Abstract

. 188:6808-6815, 2006). Here, we report that H. halophilus switches its osmolyte strategy and produces proline as the dominant solute at higher salinities (2 to 3 M NaCl). The proline biosynthesis genes proH, proJ, and proA were identified. They form a transcriptional unit and encode the pyrroline-5-carboxylate reductase, the glutamate-5-kinase, and the glutamate-5-semialdehyde dehydrogenase, respectively, catalyzing proline biosynthesis from glutamate. Expression of the genes was clearly salinity dependent and reached a maximum at 2.5 M NaCl, indicating that the pro operon is involved in salinity-induced proline biosynthesis. To address the role of anions in the process of pro gene activation and proline biosynthesis, we used a cell suspension system. Chloride salts lead to the highest accumulation of proline. Interestingly, chloride could be substituted to a large extent by glutamate salts. This unexpected finding was further analyzed on the transcriptional level. The cellular mRNA levels of all three pro genes were increased up to 90-fold in the presence of glutamate. A titration revealed that a minimal concentration of 0.2 M glutamate already stimulated pro gene expression. These data demonstrate that the solute glutamate is involved in the switch of osmolyte strategy from glutamate to proline as the dominant compatible solute during the transition from moderate to high salinity.

Biosynthesis of the Osmoprotectant Ectoine, but Not Glycine Betaine, Is Critical for Survival of Osmotically Stressed Vibrio parahaemolyticus Cells

“…The moderate halophile Halobacillus halophilus, under moderate salinities accumulates glutamate and glutamine to adjust turgor (47). In this organism, one of the major roles of glutamate is to induce proline synthesis at high-salinity conditions (48). However, in V. parahaemolyticus, the accumulation of glutamate is not sufficient for long-term survival, since the ectB mutant cannot grow in defined media under high-NaCl stress, but the betA mutant can, indicating that ectoine is the main compatible solute synthesized de novo.…”

Comparative genomics reveals adaptations of a halotolerant thaumarchaeon in the interfaces of brine pools in the Red Sea

“…This suggests the potential for osmolyte switching (Martin et al, 2001;Saum and Muller, 2007), which might offer these AOA a greater osmotolerance in a halocline environment like the BSI. PRODH genes also occur in two other thaumarchaea (Figure 3b Figure 5 Metabolic scheme of ectoine, glutamate and proline synthesis and catabolism inferred from the Nitrosopumilus genomes.…”

Ectoine and Hydroxyectoine as Protectants against Osmotic and Cold Stress: Uptake through the SigB-Controlled Betaine-Choline- Carnitine Transporter-Type Carrier EctT from Virgibacillus pantothenticus

“…Modest rises in the environmental salinity lead to moderate increases in the proline pool, whereas ectoine dominates the compatible solute pool of V. pantothenticus at higher salinities (34). This phenomenon has been termed "osmolyte switching" (50) and implies that ectoine is a more effective osmostress protectant than proline for severely salt-challenged V. pantothenticus cells.…”