A Phosphate-Bond-Driven Dipeptide Transport System in Streptococcus cremoris Is Regulated by the Internal pH

Peptide Utilization Encoded by Lactococcus lactis subsp. lactis SSL135 Chromosomal DNA

A cloned chromosomal fragment of Lactococcus lactis subsp. lactis SSL135 on plasmid p VS8 in an L. lactis subsp. lactis MG1614 background enabled proteinase-negative strain MG1614 to grow in autoclaved milk. The strain (VS230) did not, however, degrade milk proteins and did not grow in pasteurized milk. In contrast, a strain (VS150) carrying p VS9, the proteinase plasmid of SSL135, in an MG1614 background degraded beta-casein but did not grow in milk. VS230 was shown to utilize peptides produced by VS150 in growth experiments in pasteurized milk preincubated with the latter strain. The peptide utilization phenotype linked with p VS8 was further confirmed by growth of VS230 on tryptic peptide fractions, on which the plasmid-free but otherwise isogenic strain MG1614 failed to grow. Plasmid p VS8 produced 69-, 42-, 38-, and 36-kilodalton proteins, as determined by in vitro transcription-translation. At least three of these proteins affected the peptide utilization phenotype. We suggest that there could be a coupled peptidase-peptide transport system encoded by the chromosomal fragment.

The relation of proton motive force, adenylate energy charge and phosphorylation potential to the specific growth rate and efficiency of energy transduction in Bacillus licheniformis under aerobic growth conditions

The magnitude of the proton motive force (delta p) and its constituents, the electrical (delta psi) and chemical potential (-Z delta pH), were established for chemostat cultures of a protease-producing, relaxed (rel-) variant and a not protease-producing, stringent (rel+) variant of an industrial strain of Bacillus licheniformis (respectively referred to as the A- and the B-type). For both types, an inverse relation of delta p with the specific growth rate mu was found. The calculated intracellular pH (pHin) was not constant but inversely related to mu. This change in pHin might be related to regulatory functions of metabolism but a regulatory role for pHin itself could not be envisaged. Measurement of the adenylate energy charge (EC) showed a direct relation with mu for glucose-limited chemostat cultures; in nitrogen-limited chemostat cultures, the EC showed an approximately constant value at low mu and an increased value at higher mu. For both limitations, the ATP/ADP ratio was directly related to mu. The phosphorylation potential (delta G'p) was invariant with mu. From the values for delta G'p and delta p, a variable -->H+/ATP-stoichiometry was inferred: -->H+/ATP = 1.83 +/- 0.52 mu, so that at a given -->H+/O-ratio of four (4), the apparent P/O-ratio (inferred from regression analysis) showed a decline of 2.16 to 1.87 for mu = 0 to mu max (we discuss how more than half of this decline will be independent of any change in internal cell-volume). We propose that the constancy of delta G'p and the decrease in the efficiency of energy-conservation (P/O-value) with increasing mu are a way in which the cells try to cope with an apparent less than perfect coordination between anabolism and catabolism to keep up the highest possible mu with a minimum loss of growth-efficiency. Protease production in nitrogen-limited cultures as compared to glucose-limited cultures, and the difference between the A- and B-type, could not be explained by a different energy-status of the cells.

Bioenergetic consequences of catabolic shifts by Lactobacillus plantarum in response to shifts in environmental oxygen and pH in chemostat cultures

Proton motive force (PMF), intracellular end product concentrations, and ATP levels were determined when a steady-state Lactobacillus plantarum 8014 anaerobic chemostat culture was shifted to an aerobic condition or was shifted from pH 5.5 to 7.5. The PMF and intracellular ATP levels increased immediately after the culture was shifted from anaerobic to aerobic conditions. The concentrations of intracellular lactate and acetate, which exported protons that contributed to the proton gradient, changed in the same fashion. The H+/lactate stoichiometry, n, varied from 0.8 to 1.2, and the H+/acetate n value changed from 0.8 to 1.6 at 2 h after the shift to aerobic conditions. The n value for acetate excretion remained elevated at aerobic steady state. When the anaerobic culture was shifted from pH 5.5 to 7.5, intracellular ATP increased 20% immediately even though the PMF decreased 50% as a result of the depletion of the transmembrane proton gradient. The H+/lactate n value changed from 0.7 to 1.8, and n for H+/acetate increased from 0.9 to 1.9 at pH 7.5 steady state. In addition, the H+/acetate stoichiometry was always higher than the n value for H+/lactate; both were higher in alkaline than aerobic conditions, demonstrating that L. plantarum 8014 coexcreted more protons with end products to maintain intracellular pH homeostasis and generate proton gradients under aerobic and alkaline conditions. During the transient to pH 7.5, the n value for H+/acetate approached 3, which would spare one ATP.

Uptake of glutamate in Corynebacterium glutamicum. 2. Evidence for a primary active transport system

The inducible glutamate uptake system in Corynebacterium glutamicum (Krämer, R., Lambert, C., Hoischen, C. & Ebbighausen, H., preceding paper in this journal) was characterized with respect to its mechanism and energy coupling. All possible secondary active uptake mechanisms can be excluded. Glutamate transport is not coupled to the translocation of H+, Na+ or K+ ions. Although changes in membrane potential and uptake activity cannot completely be separated, no correlation between these two parameters is observed. The uptake of glutamate resembles a primary active, ATP-dependent transport mechanism in several respects. (a) The substrate affinity is very high (1.3 microM). (b) Accumulation of glutamate reaches values of greater than 2.10(5), at least as high as those reported for binding-protein-dependent systems in Gram-negative bacteria. (c) The uptake is unidirectional. Even after complete deenergization, the accumulation ratio was not significantly reduced. (d) The rate of glutamate uptake is directly correlated to the cytosolic ATP content and also to the ATP/ADP ratio. This is shown by varying internal ATP by different procedures applying inhibitors (NaCN, dicyclohexyl carbodiimide), uncouplers (carbonyl m-chlorophenylhydrazone), ionophores (valinomycin), and even by shifting the cells to anaerobiosis. Uptake is not promoted by cytosolic ATP levels below 1.5 mM, the maximum uptake rate is reached at 4-5 mM ATP.

Genetics of the proteolytic system of lactic acid bacteria

The proteolytic system of lactic acid bacteria is of eminent importance for the rapid growth of these organisms in protein-rich media. The combined action of proteinases and peptidases provides the cell with small peptides and essential amino acids. The amino acids and peptides thus liberated have to be translocated across the cytoplasmic membrane. To that purpose, the cell contains specific transport proteins. The internalized peptides are further degraded to amino acids by intracellular peptidases. The world-wide economic importance of the lactic acid bacteria and their proteolytic system has led to an intensive research effort in this area and a considerable amount of biochemical data has been collected during the last two decades. Since the development of systems to genetically manipulate lactic acid bacteria, data on the genetics of enzymes and processes involved in proteolysis are rapidly being generated. In this review an overview of the latest genetic data on the proteolytic system of lactic acid bacteria will be presented. As most of the work in this field has been done with lactococci, the emphasis will, inevitably, be on this group of organisms. Where possible, links will be made with other species of lactic acid bacteria.

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris Wg2

An aminopeptidase was purified to homogeneity from a crude cell extract of Lactococcus lactis subsp. cremoris Wg2 by a procedure that included diethyl-aminoethane-Sephacel chromatography, phenyl-Sepharose chromatography, gel filtration, and high-performance liquid chromatography over an anion-exchange column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band with a molecular weight of 95,000. The aminopeptidase was capable of degrading several peptides by hydrolysis of the N-terminal amino acid. The peptidase had no endopeptidase or carboxypeptidase activity. The aminopeptidase activity was optimal at pH 7 and 40�C. The enzyme was completely inactivated by the p -chloromecuribenzoate mersalyl, chelating agents, and the divalent cations Cu 2+ and Cd 2+ . The activity that was lost by treatment with the sulfhydryl-blocking reagents was restored with dithiothreitol or β-mercapto-ethanol, while Zn 2+ or Co 2+ restored the activity of the 1,10-phenantroline-treated enzyme. Kinetic studies indicated that the enzyme has a relatively low affinity for lysyl- p -nitroanilide ( K m , 0.55 mM) but that it can hydrolyze this substrate at a high rate ( V max , 30 μmol/min per mg of protein).

Peptide uptake is essential for growth of Lactococcus lactis on the milk protein casein

The chlorated dipeptide L-alanyl-beta-chloro-L-alanine (diACA) is very toxic for Lactococcus lactis. Spontaneous mutants resistant to the dipeptide were isolated from plates. The presence and activities of cell wall-associated proteinase, different peptidases in cell extracts, amino acid transport systems, and di- and oligopeptide transport systems were examined and compared in a diACA-resistant mutant and the wild type. Only the rates of di- and tripeptide transport were found to be significantly reduced in the diACA-resistant mutant of L. lactis ML3. Since all other characteristics of this mutant were comparable to those of the wild type, the diACA-resistant mutant is most likely deficient in di- and tripeptide transport. Uptake of di- and tripeptides by L. lactis ML3 was found to be mainly mediated by one peptide transport system. The peptide transport-deficient mutant was found to be unable to grow on a chemically defined medium supplemented with casein as the sole nitrogen source, whereas growth could be restored by the addition of amino acids. These results indicate that peptide transport in L. lactis ML3 is an essential component in the process of casein utilization during growth in milk.

Mechanism and energetics of dipeptide transport in membrane vesicles of Lactococcus lactis

Alanyl-alpha-glutamate transport has been studied in Lactococcus lactis ML3 cells and in membrane vesicles fused with liposomes containing beefheart cytochrome c oxidase as a proton-motive-force-generating system. The uptake of Ala-Glu observed in de-energized cells can be stimulated 26-fold upon addition of lactose. No intracellular dipeptide pool could be detected in intact cells. In fused membranes, a 40-fold accumulation of Ala-Glu was observed in response to a proton motive force. Addition of ionophores and uncouplers resulted in a rapid efflux of the accumulated dipeptide, indicating that Ala-Glu accumulation is directly coupled to the proton motive force as a driving force. Ala-Glu uptake is an electrogenic process and the dipeptide is transported in symport with two protons. In both fused membranes and intact cells the same affinity constant (0.70 mM) for Ala-Glu uptake was found. Accumulated Ala-Glu is exchangeable with externally added alanyl-glutamate, glutamyl-glutamate, and leucyl-leucine, while no exchange occurred upon addition of the amino acid glutamate or alanine. These results indicate that the Ala-Glu transport system has a broad substrate specificity.

Bioenergetics and solute transport in lactococci

During the last few years the studies about the physiology and bioenergetics of lactic acid bacteria during growth and starvation have evolved from a descriptive level to an analysis of the molecular events in the regulation of various processes. Considerable progress has been made in the understanding of the modes of metabolic energy generation, the mechanism of homeostasis of the internal pH, and the mechanism and regulatory processes of transport systems for sugars, amino acids, peptides, and ions. Detailed studies of these transport processes have been performed in cytoplasmic membrane vesicles of these organisms in which a foreign proton pump has been introduced to generate a high proton motive force.

Sodium-dependent transport of neutral amino acids by whole cells and membrane vesicles of Streptococcus bovis, a ruminal bacterium

Streptococcus bovis JB1 cells were able to transport serine, threonine, or alanine, but only when they were incubated in sodium buffers. If glucose-energized cells were washed in potassium phosphate and suspended in potassium phosphate buffer, there was no detectable uptake. Cells deenergized with 2-deoxyglucose and incubated in sodium phosphate buffer were still able to transport serine, and this result indicated that the chemical sodium gradient was capable of driving transport. However, when the deenergized cells were treated with valinomycin and diluted into sodium phosphate to create both an artificial membrane potential and a chemical sodium gradient, rates of serine uptake were fivefold greater than in cells having only a sodium gradient. If deenergized cells were preloaded with sodium (no membrane potential or sodium gradient), there was little serine transport. Nigericin and monensin, ionophores capable of reversing sodium gradients across membranes, strongly inhibited sodium-dependent uptake of the three amino acids. Membrane vesicles loaded with potassium and diluted into either lithium or choline chloride were unable to transport serine, but rapid uptake was evident if sodium chloride was added to the assay mixture. Serine transport had an extremely poor affinity for sodium, and more than 30 mM was needed for half-maximal rates of uptake. Serine transport was inhibited by an excess of threonine, but an excess of alanine had little effect. Results indicated that S. bovis had separate sodium symport systems for serine or threonine and alanine, and either the membrane potential or chemical sodium gradient could drive uptake.