Nonspecific inhibition of proline dehydrogenase synthesis in Escherichia coli during osmotic stress

Osmotic Stress

Escherichia coli and Salmonella encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. E. coli and Salmonella limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K+ accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K+ glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as E. coli and Salmonella survive or thrive in their natural environments.

Proline utilization by Bacillus subtilis: uptake and catabolism

L-Proline can be used by Bacillus subtilis as a sole source of carbon or nitrogen. We traced L-proline utilization genetically to the putBCP (ycgMNO) locus. The putBCP gene cluster encodes a high-affinity proline transporter (PutP) and two enzymes, the proline dehydrogenase PutB and the Δ(1)-pyrroline-5-carboxylate dehydrogenase PutC, which jointly catabolize L-proline to L-glutamate. Northern blotting, primer extension, and putB-treA reporter gene fusion analysis showed that the putBCP locus is transcribed as an L-proline-inducible operon. Its expression was mediated by a SigA-type promoter and was dependent on the proline-responsive PutR activator protein. Induction of putBCP expression was triggered by the presence of submillimolar concentrations of L-proline in the growth medium. However, the very large quantities of L-proline (up to several hundred millimolar) synthesized by B. subtilis as a stress protectant against high osmolarity did not induce putBCP transcription. Induction of putBCP transcription by external L-proline was not dependent on L-proline uptake via the substrate-inducible PutP or the osmotically inducible OpuE transporter. It was also not dependent on the chemoreceptor protein McpC required for chemotaxis toward L-proline. Our findings imply that B. subtilis can distinguish externally supplied L-proline from internal L-proline pools generated through de novo synthesis. The molecular basis of this regulatory phenomenon is not understood. However, it provides the B. subtilis cell with a means to avoid a futile cycle of de novo L-proline synthesis and consumption by not triggering the expression of the putBCP L-proline catabolic genes in response to the osmoadaptive production of the compatible solute L-proline.

Upregulation of ATP-sensitive potassium channels for estrogen-mediated cell proliferation in human uterine leiomyoma cells

OBJECTIVES

The objectives of the present study were to evaluate the expression level of ATP-sensitive potassium (K(ATP)) channels in smooth muscle cells in human uterine leiomyoma and the involvement of the channel in potentiating effect of estrogen on leiomyoma growth.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR), real-time PCR and Western blot were used for the identification and quantification of K(ATP)-channel subunits in the control myometrial and leiomyoma cells. Furthermore, we measured the K(ATP)-channel activity in enzymatically isolated single uterine smooth muscle cells by whole-cell patch-clamp recordings. The estrogen-induced cell proliferation in leiomyoma was measured by the MTT assay.

RESULTS

The subunits of K(ATP) channels (Kir6.1, Kir6.2, SUR2B) were more highly expressed in leiomyoma cells than in control cells. The whole-cell currents mainly through K(ATP) channels were also greater in the leiomyoma cells. Estrogen applied in the bath solution could acutely enhance the channel activity. Estrogen-induced proliferation of the leiomyoma cells was inhibited by pretreatment with glibenclamide, a K(ATP)-channel inhibitor.

CONCLUSION

Estrogen may induce the proliferation of leiomyoma cells, at least in part, by activating the K(ATP) channel. Increased expression of the K(ATP) channel may be a causal factor for the high growth rate of uterine leiomyoma.

Osmoprotection of Escherichia coli by peptone is mediated by the uptake and accumulation of free proline but not of proline-containing peptides

The effect of meat peptone type I (Sigma) on the growth of Escherichia coli cells under hyperosmotic stress has been investigated. Peptone is a complex mixture of peptides with a small content of free amino acids, which resembles nutrients found in natural environments. Our data showed that peptone enhances the growth of E. coli cells in high-osmolarity medium to levels higher than those achieved with the main compatible solute in bacteria, glycine betaine. The mechanism of osmoprotection by peptone comprises the uptake and accumulation of the compatible solute, proline. The main role of the peptides contained in peptone is the provision of nutrients rather than the intracellular accumulation of osmolytes. In contrast to Listeria monocytogenes (M. R. Amezaga, I. Davidson, D. McLaggan, A. Verheul, T. Abee, and I. R. Booth, Microbiology 141:41-49, 1995), E. coli does not accumulate exogenous peptides for osmoprotection and peptides containing proline do not lead to the accumulation of proline as a compatible solute. In late-logarithmic-phase cultures of E. coli growing at high osmolarity plus peptone, proline becomes the limiting factor for growth, and the intracellular pools of proline are not maintained. This is a consequence of the low concentration of free proline in peptone, the catabolism of proline by E. coli, and the inability of E. coli to utilize proline-containing peptides as a source of compatible solutes. Our data highlight the role that natural components in food such as peptides play in undermining food preservation regimes, such as high osmolarity, and also that the specific mechanisms of osmoprotection by these compounds differ according to the organism.

Osmoadaptation by rhizosphere bacteria

The osmolality of rhizosphere soil water is expected to be elevated in relation to bulk-soil water osmolality as a result of the exclusion of solutes by plant roots during water uptake, the release of plant root exudates, and the production of exopolymers by plant roots and rhizobacteria. In contrast, the osmolality of water within highly hydrated bulk soil is low (less than 50 Osm/kg); thus the ability to adapt to elevated osmolality is likely to be important for successful rhizosphere colonization by rhizobacteria. The present review focuses on the osmoadaptive responses of three gram-negative rhizobacterial genera: Rhizobium, Azospirillum, and Pseudomonas. Specifically, we examine the compatible solutes and osmoprotectants utilized by various species within these genera. The adaptation of rhizobacteria to hypoosmotic environments is also examined in the present review. In particular, we focus on the biosynthesis and accumulation of periplasmic glucans by rhizobacteria. Finally, the relationship between rhizobacterial osmoadaptation and selected plant-microbe interactions is considered.

Sensitivity of Escherichia coli to proline analogues during osmotic stress and anaerobiosis

The sensitivity of wild-type Escherichia coli K-12 to a series of proline analogues was determined in cultures containing increasing concentrations of NaCl under both aerobic and anaerobic conditions. The bacteria were most sensitive to L-azetidine-2-carboxylate and L-thiazolidine-4-carboxylate. The minimum inhibitory concentrations for these compounds decreased progressively during osmotic stress, but the bacteria were much more sensitive to these proline analogues under aerobic conditions than during anaerobiosis. The reduced sensitivity under anaerobic conditions did not reflect degradation of the compounds in the culture medium. Since both urine and medullary renal tissue contain relatively low oxygen concentrations, these results raise doubts about the potential use of proline or glycine betaine analogues in treating urinary tract infections.

Proline transport in Staphylococcus aureus: a high-affinity system and a low-affinity system involved in osmoregulation

L-Proline enhanced the growth of Staphylococcus aureus in high-osmotic-strength medium, i.e., it acted as an osmoprotectant. Study of the kinetics of L-[14C]proline uptake by S. aureus NCTC 8325 revealed high-affinity (Km = 1.7 microM; maximum rate of transport [Vmax] = 1.1 nmol/min/mg [dry weight]) and low-affinity (Km = 132 microM; Vmax = 22 nmol/min/mg [dry weight]) transport systems. Both systems were present in a proline prototrophic variant grown in the absence of proline, although the Vmax of the high-affinity system was three to five times higher than that of the high-affinity system in strain 8325. Both systems were dependent on Na+ for activity, and the high-affinity system was stimulated by lower concentrations of Na+ more than the low-affinity system. The proline transport activity of the low-affinity system was stimulated by increased osmotic strength. The high-affinity system was highly specific for L-proline, whereas the low-affinity system showed a broader substrate specificity. Glycine betaine did not compete with proline for uptake through either system. Inhibitor studies confirmed that proline uptake occurred via Na(+)-dependent systems and suggested the involvement of the proton motive force in creating an Na+ gradient. Hyperosmotic stress (upshock) of growing cultures led to a rapid and large uptake of L-[14C]proline that was not dependent on new protein synthesis. It is suggested that the low-affinity system is involved in adjusting to increased environmental osmolarity and that the high-affinity system may be involved in scavenging low concentrations of proline.