Stimulatory effect of lithium ion on proline transport by whole cells of Escherichia coli

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Proline catabolism by Pseudomonas putida: cloning, characterization, and expression of the put genes in the presence of root exudates

Pseudomonas putida KT2442 is a root-colonizing strain which can use proline, one of the major components in root exudates, as its sole carbon and nitrogen source. A P. putida mutant unable to grow with proline as the sole carbon and nitrogen source was isolated after random mini-Tn5-Km mutagenesis. The mini-Tn5 insertion was located at the putA gene, which is adjacent to and divergent from the putP gene. The putA gene codes for a protein of 1,315 amino acid residues which is homologous to the PutA protein of Escherichia coli, Salmonella enterica serovar Typhimurium, Rhodobacter capsulatus, and several Rhizobium strains. The central part of P. putida PutA showed homology to the proline dehydrogenase of Saccharomyces cerevisiae and Drosophila melanogaster, whereas the C-terminal end was homologous to the pyrroline-5-carboxylate dehydrogenase of S. cerevisiae and a number of aldehyde dehydrogenases. This suggests that in P. putida, both enzymatic steps for proline conversion to glutamic acid are catalyzed by a single polypeptide. The putP gene was homologous to the putP genes of several prokaryotic microorganisms, and its gene product is an integral inner-membrane protein involved in the uptake of proline. The expression of both genes was induced by proline added in the culture medium and was regulated by PutA. In a P. putida putA-deficient background, expression of both putA and putP genes was maximal and proline independent. Corn root exudates collected during 7 days also strongly induced the P. putida put genes, as determined by using fusions of the put promoters to 'lacZ. The induction ratio for the putA promoter (about 20-fold) was 6-fold higher than the induction ratio for the putP promoter.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

Ab initio studies of the decomposition of nitrosourea in the presence of cations

Self-consistent (Hartree-Fock) calculations of the process of decomposition of protonated and lithiated syn-N-nitrosourea show that the presence of cations perturbs the electron distribution significantly. The decomposition of nitrosourea is facilitated when a proton or lithium ion is positioned at the oxygen of the nitroso group. These results may suggest clinical experimentation with nitrosoureas used in conjunction with lithium salts.

Osmotic regulation of L-proline transport in Salmonella typhimurium

The transport of proline is important for the adaptation of Salmonella typhimurium to osmotic stress because exogenous proline permits the growth of the organism in media of elevated osmotic strength that would otherwise be toxic. Measurements of the rate of [3H]proline transport in S. typhimurium indicated that the organism has two distinct proline permeases, the ProU and the ProP systems, whose activities increase more than fivefold as a consequence of growth in media containing 0.3 M NaCl or 0.47 M sucrose. Transport via a third proline permease, the PutP system, is not affected by the osmotic strength of the medium. We constructed strains that carry fusions of lacZ to proU or proP, genes that are required for the two osmotically stimulated proline transport systems. Assays of beta-galactosidase revealed that the transcription of the proU gene is increased more than 10-fold as a result of exposure of the cells to 0.3 M NaCl, 0.47 M sucrose, or equivalent concentrations of other solutes that are not freely diffusible across the cytoplasmic membrane. Increased transcription cannot be triggered by methanol, ethanol, and glycerol, substances that are freely diffusible across the membrane, suggesting that the signal for increased transcription might be an osmotic gradient across the cytoplasmic membrane. The proP gene does not show transcriptional regulation of sufficient magnitude to account for the stimulation of [3H]proline transport. Thus, the osmotic stimulation of the ProP system might be mediated by some posttranscriptional event.

Na+ (Li+)-proline cotransport in Escherichia coli

Na+ and Li+ were found to stimulate the transport of L-proline by cells of Escherichia coli induced for proline utilization. The gene product of the put P gene is involved in the expression of this transport activity since the put P+ strains CSH 4 and WG 148 show activity and the put P- strain RM 2 fails to show this cation coupled transport. The addition of proline was found to stimulate the uptake of Li+ and of Na+. Attempts to demonstrate proline stimulated H+ uptake were unsuccessful. It is concluded that the proline carrier (coded by the put P gene) is responsible for Na+ (or Li+)-proline cotransport.

Proline uptake through the major transport system of Salmonella typhimurium is coupled to sodium ions

Strains of Salmonella typhimurium deficient in one or more of the proline transport systems have been constructed and used to study the mechanism of energy coupling to transport. Proline uptake through the major proline permease (PP-I, putP) is shown to be absolutely coupled to Na+ ions and not to H+ ions as has previously been assumed. Transport through the minor proline permease (PP-II, proP), however, is unaffected by the presence or absence of Na+. The effect of Na+ on the kinetics of proline uptake shows that external Na+ increases the Vmax for transport. It seems probable that proline transport through PP-I is also coupled to Na+ ions in Escherichia coli.

Cotransport of proline and Li+ in Escherichia coli

Uptake of Li+ induced by the addition of proline to a cell suspension of Escherichia coli was detected using an Li+-selective electrode. This Li+ uptake was inhibited by L-azetidine 2-carboxylic acid, a competitive inhibitor of the proline transport system. Thus, direct evidence for Li+-proline cotransport via the proline transport system was obtained. Kinetic parameters of the Li+ uptake were determined.

The effect of lithium chloride on tumour appearance and survival of melanoma-bearing mice

The possible effect of lithium chloride, a compound which reduces the incidence of infection in cancer patients, was investigated on murine melanoma. C57 BL syngeneic mice were inoculated i.p. with B16 melanoma cells. The animals were divided into 4 groups, receiving daily i.p. treatment with saline--group 1, controls; lithium chloride--group 2, bleomycin and vinblastine--group 3, and lithium chloride with bleomycin and vinblastine--group 4. Animals in group 4 had a significant delay in tumour appearance, a higher degree of tumour necrosis, and a longer survival rate. In addition a significant reduction of serum lithium concentration was noted in animals of this group in comparison with animals in group 2, treated with lithium chloride alone. There was no lithium-induced leukocytosis.

Proline over-production results in enhanced osmotolerance in Salmonella typhimurium

Mutants of S. typhimurium with enhanced osmotolerance were isolated. These mutants were obtained as strains which over-produced proline due to regulatory mutations affecting proline biosynthesis. The mutations are located on F'proBa and upon transfer to other S. typhimurium strains, they confer enhanced osmotolerance on the recipients. The osmotolerant mutants not only have higher intracellular proline levels than the osmosensitive parental strain, but the proline levels in the osmotolerant mutants are regulated such that they increase in response to osmotic stress. Possibly reasons why elevated proline levels lead to enhanced osmotolerance are discussed.

Proton-linked D-xylose transport in Escherichia coli

The addition of xylose to energy-depleted cells of Escherichia coli elicited an alkaline pH change which failed to appear in the presence of uncoupling agents. Accumulation of [14C]xylose by energy-replete cells was also inhibited by uncoupling agents, but not by fluoride or arsenate. Subcellular vesicles of E. coli accumulated [14C]xylose provided that ascorbate plus phenazine methosulfate were present for respiration, and this accumulation was inhibited by uncoupling agents or valinomycin. Therefore, the transport of xylose into E. coli appears to be energized by a proton-motive force, rather than by a phosphotransferase or directly energized mechanism. Its specificity for xylose as inducer and substrate and the genetic location of a xylose-H+ transport-negative mutation near mtl showed that the xylose-H+ system is distinct from other proton-linked sugar transport systems of E. coli.

Role of lithium ions in proline transport in Escherichia coli

Mechanisms of Li+ stimulation of proline transport were studied in cells of Escherichia coli 7 and NR70, a mutant of strain 7 lacking adenosine triphosphatase (EC 3.6.1.3). An electrochemical potential difference of Li+ induced in an inward direction of energy-depleted cells caused a transient uptake of proline depending on the driving force provided. When proline was added to unbuffered cell suspensions under anaerobic conditions, the medium was found to be acidified only in the presence of Li+ but not in the presence of Na+ or K+. This acidification was abolished by the addition of a permeant anion, SCN-, to the medium containing Li+, but this was not demonstrated with cells of a mutant strain deficient in a carrier protein specific for proline. These results support the assumption that proline is taken up by a mechanism of Li+-proline cotransport in E. coli.

Role of Na+ and Li+ in thiomethylgalactoside transport by the melibiose transport system of Escherichia coli

Thiomethyl-beta-galactoside (TMG) accumulation via the melibiose transport system was studied in lactose transport-negative strains of Escherichia coli. TMG uptake by either intact cells or membrane vesicles was markedly stimulated by Na+ or Li+ between pH 5.5 and 8. The Km for uptake of TMG was approximately 0.2 mM at an external Na+ concentration of 5 mM (pH 7). The alpha-galactosides, melibiose, methyl-alpha-galactoside, and o-nitrophenyl-alpha-galactoside had a high affinity for this system whereas lactose, maltose and glucose had none. Evidence is presented for Li+-TMG or Na+-TMG cotransport.