). The second system is a metabolic energy-dependent oligopeptide transport system which transports peptides of four to at least six amino acid residues. The involvement of a specific oligopeptide transport system in the utilization of tetra-alanine and penta-alanine was established in a mutant of L. lactis MG1363 that was selected on the basis of resistance to toxic analogs of alanine and alanine-containing di-and tripeptides. This mutant is unable to transport alanine, dialanine, and trialanine but still shows uptake of tetra-alanine and penta-alanine. The oligopeptide transport system has a lower activity than the di-tripeptide transport system. Uptake of oligopeptides occurs in the absence of a proton motive force and is specifically inhibited by vanadate. The oligopeptide transport system is most likely driven by ATP or a related energy-rich, phosphorylated intermediate.Lactococci require, in addition to a carbohydrate source, nucleotides, vitamins (3), and amino acids (20) for growth. The actual number of amino acids required for growth is strain dependent. The amino acid requirement can be satisfied by free amino acids, peptides, and/or proteins (i.e., caseins). For degradation of proteins, lactococci possess an extracellular cell wall-bound proteinase and peptidases which act in concert to supply the cells with essential and growth-stimulating amino acids and small peptides (for reviews, see references 9 and 26). Transport of the caseinderived amino acids is mediated by several different amino acid transport systems (for a review, see reference 7).In addition to amino acid uptake, lactococci can also satisfy their amino acid demand by uptake of peptides. One uptake system specific for di-and tripeptides has been investigated in a peptidase-free membrane vesicle system (23). In that study, alanyl-glutamate (Ala-Glu) was used as a model substrate. Accumulation of the dipeptide in membrane vesicles of Lactococcus lactis ML3 fused with liposomes containing beef heart cytochrome c oxidase was found to be driven by the electrical potential (A+f) and the chemical gradient of protons (ApH) across the membrane (23). Information about the specificity of this transport system is limited and is restricted to results obtained from competition experiments in which the rate of uptake of radioactively labelled peptides was estimated in the absence or presence of a few unlabelled peptides (10,22,23 in-derived amino acids will be supplied to the cells as proline-containing peptides. The finding that especially proline-containing dipeptides are high-affinity substrates for the lactococcal di-tripeptide transport system is in agreement with this notion (24).Information about the presence and properties of transport systems for oligopeptides (peptides containing four or more amino acid residues) is virtually lacking. Growth (10) and transport (22) studies with L. lactis have indicated that the size restriction for peptide utilization is four to five amino acid residues. In this study, the presence of an oligopeptide transpor...

In Streptococcus lactis ML3 and Streptococcus cremoris Wg2 the uptake of glutamate and glutamine is mediated by the same transport system, which has a 30-fold higher affinity for glutamine than for glutamate at pH 6.0. The apparent affinity constant for transport (KT) of glutamine is 2.5 0.3 ,uM, independent of the extracellular pH. The KTs for glutamate uptake are 3.5, 11.2, 77, and 1200 ,IM at pH 4.0, 5.1, 6.0, and 7.0, respectively. Recalculation of the affinity constants based on the concentration of glutamic acid in the solution yield KTs of 1.8 + 0.5 ,uM independent of the external pH, indicating that the protonated form of glutamate, i.e., glutamic acid, and glutamine are the transported species. The maximal rates of glutamate and glutamine uptake are independent of the extracellular pH as long as the intracellular pH is kept constant, despite large differences in the magnitude and composition of the components of the proton motive force. Uptake of glutamate and glutamine requires the synthesis of ATP either from glycolysis or from arginine metabolism and appears to be essentially unidirectional. Cells are able to maintain glutamate concentration gradients exceeding 4 x 103 for several hours even in the absence of metabolic energy. The t1/2s of glutamate efflux are 2, 12, and >30 h at pH 5.0, 6.0, and 7.0, respectively. After the addition of lactose as energy source, the rate of glutamine uptake and the level of ATP are both very sensitive to arsenate. When the intracellular pH is kept constant, both parameters decrease approximately in parallel (between 0.2 and 1.0 mM ATP) with increasing concentrations of the inhibitor. These results suggest that the accumulation of glutamate and glutamine is energized by ATP or an equivalent energy-rich phosphorylated intermediate and not by the proton motive force.With a few exceptions, streptococci are nutritionally fastidious, which means that many solutes have to be translocated from the medium into the cytoplasm. Detailed information is available about the regulation and energy coupling of sugar transport systems in streptococci (Streptococcus cremoris, S. lactis, and S. diacetylactis) (21). Studies on amino acid transport (1,11,20,27) and ion movements (3, 10, 13) have been performed in S. faecalis and more recently in S. cremoris (8,18). In S. faecalis the operation of two kinetically distinct dicarboxylic acid transport systems has been shown with affinities for glutamate and aspartate (20). Transport of aspartate and glutamate by the high-affinity system appears to be essentially unidirectional and requires ATP but not the proton motive force (11). The transport system for a-aminoisobutyric acid, a nonmetabolizable analog of alanine, and a number of structurally similar neutral amino acids such as serine, alanine, and glycine has been studied in S. lactis ML3; this system is most likely driven by the proton motive force (26). The proton motive force has also been shown to be the driving force for the accumulation of (neutral) amino acids in S. faecalis (1).The...

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