Amino acid transport in Mycobacterium smegmatis

Eukaryotic pathways targeted by the type III secretion system effector protein, BipC, involved in the intracellular lifecycle of Burkholderia pseudomallei

Burkholderia pseudomallei, the etiological agent for melioidosis, is known to secrete a type III secretion system (TTSS) protein into the host’s internal milieu. One of the TTSS effector protein, BipC, has been shown to play an important role in the B. pseudomallei pathogenesis. To identify the host response profile that was directly or indirectly regulated by this protein, genome-wide transcriptome approach was used to examine the gene expression profiles of infected mice. The transcriptome analysis of the liver and spleen revealed that a total of approximately 1,000 genes were transcriptionally affected by BipC. Genes involved in bacterial invasion, regulation of actin cytoskeleton, and MAPK signalling pathway were over-expressed and may be specifically regulated by BipC in vivo. These results suggest that BipC mainly targets pathways related to the cellular processes which could modulate the cellular trafficking processes. The host transcriptional response exhibited remarkable differences with and without the presence of the BipC protein. Overall, the detailed picture of this study provides new insights that BipC may have evolved to efficiently manipulate host-cell pathways which is crucial in the intracellular lifecycle of B. pseudomallei.

Investigation of the essentiality of glutamate racemase in Mycobacterium smegmatis

The mycobacterial cell wall frequently has been used as a target for drug development, and d-glutamate, synthesized by glutamate racemase (MurI), is an important component of peptidoglycan. While the essentiality of the murI gene has been shown in several bacterial species, including Escherichia coli, Bacillus anthracis, and Streptococcus pneumoniae, studies in mycobacteria have not yet provided definitive results. This study aimed to determine whether murI is indeed essential and can serve as a possible target for structure-aided drug design. We have achieved this goal by creating a ΔmurI strain of Mycobacterium smegmatis, a close relative of Mycobacterium tuberculosis. The deletion of the murI gene in M. smegmatis could be achieved only in minimal medium supplemented with D-glutamate, demonstrating that MurI is essential for growth and that glutamate racemase is the only source of D-glutamate for peptidoglycan synthesis in M. smegmatis.

Physiology of mycobacteria

Mycobacterium tuberculosis is a prototrophic, metabolically flexible bacterium that has achieved a spread in the human population that is unmatched by any other bacterial pathogen. The success of M. tuberculosis as a pathogen can be attributed to its extraordinary stealth and capacity to adapt to environmental changes throughout the course of infection. These changes include: nutrient deprivation, hypoxia, various exogenous stress conditions and, in the case of the pathogenic species, the intraphagosomal environment. Knowledge of the physiology of M. tuberculosis during this process has been limited by the slow growth of the bacterium in the laboratory and other technical problems such as cell aggregation. Advances in genomics and molecular methods to analyze the M. tuberculosis genome have revealed that adaptive changes are mediated by complex regulatory networks and signals, resulting in temporal gene expression coupled to metabolic and energetic changes. An important goal for bacterial physiologists will be to elucidate the physiology of M. tuberculosis during the transition between the diverse conditions encountered by M. tuberculosis. This review covers the growth of the mycobacterial cell and how environmental stimuli are sensed by this bacterium. Adaptation to different environments is described from the viewpoint of nutrient acquisition, energy generation, and regulation. To gain quantitative understanding of mycobacterial physiology will require a systems biology approach and recent efforts in this area are discussed.

Nutrient acquisition by mycobacteria

The growth and nutritional requirements of mycobacteria have been intensively studied since the discovery of Mycobacterium tuberculosis more than a century ago. However, the identity of many transporters for essential nutrients of M. tuberculosis and other mycobacteria is still unknown despite a wealth of genomic data and the availability of sophisticated genetic tools. Recently, considerable progress has been made in recognizing that two lipid permeability barriers have to be overcome in order for a nutrient molecule to reach the cytoplasm of mycobacteria. Uptake processes are discussed by comparing M. tuberculosis with Mycobacterium smegmatis. For example, M. tuberculosis has only five recognizable carbohydrate transporters in the inner membrane, while M. smegmatis has 28 such transporters at its disposal. The specificities of inner-membrane transporters for sulfate, phosphate and some amino acids have been determined. Outer-membrane channel proteins in both organisms are thought to contribute to nutrient uptake. In particular, the Msp porins have been shown to be required for uptake of carbohydrates, amino acids and phosphate by M. smegmatis. The set of porins also appears to be different for M. tuberculosis and M. smegmatis. These differences likely reflect the lifestyles of these mycobacteria and the availability of nutrients in their natural habitats: the soil and the human body. The comprehensive identification and the biochemical and structural characterization of the nutrient transporters of M. tuberculosis will not only promote our understanding of the physiology of this important human pathogen, but might also be exploited to improve tuberculosis chemotherapy.

Amino acid transport and metabolism in mycobacteria: cloning, interruption, and characterization of an L-Arginine/gamma-aminobutyric acid permease in Mycobacterium bovis BCG

Genes encoding L-arginine biosynthetic and transport proteins have been shown in a number of pathogenic organisms to be important for metabolism within the host. In this study we describe the cloning of a gene (Rv0522) encoding an amino acid transporter from Mycobacterium bovis BCG and the effects of its deletion on L-arginine transport and metabolism. The Rv0522 gene of BCG was cloned from a cosmid library by using primers homologous to the rocE gene of Bacillus subtilis, a putative arginine transporter. A deletion mutant strain was constructed by homologous recombination with the Rv0522 gene interrupted by a selectable marker. The mutant strain was complemented with the wild-type gene in single copy. Transport analysis of these strains was conducted using (14)C-labeled substrates. Greatly reduced uptake of L-arginine and gamma-aminobutyric acid (GABA) but not of lysine, ornithine, proline, or alanine was observed in the mutant strain compared to the wild type, grown in Middlebrook 7H9 medium. However, when the strains were starved for 24 h or incubated in a minimal salts medium containing 20 mM arginine (in which even the parent strain does not grow), L-[(14)C]arginine uptake by the mutant but not the wild-type strain increased strongly. Exogenous L-arginine but not GABA, lysine, ornithine, or alanine was shown to be toxic at concentrations of 20 mM and above to wild-type cells growing in optimal carbon and nitrogen sources such as glycerol and ammonium. L-Arginine supplied in the form of dipeptides showed no toxicity at concentrations as high as 30 mM. Finally, the permease mutant strain showed no defect in survival in unactivated cultured murine macrophages compared with wild-type BCG.

Permeability barrier to hydrophilic solutes in Mycobacterium chelonei

In order to define the permeability barrier to hydrophilic molecules in mycobacteria, we used as a model a smooth, beta-lactamase-producing strain of Mycobacterium chelonei. The rates of hydrolysis of eight cephalosporins by intact and sonicated cells were measured, and the permeability coefficient (P) was calculated from these rates by the method of Zimmermann and Rosselet (W. Zimmermann and A. Rosselet, Antimicrob. Agents Chemother. 12:368-372, 1977). P ranged from (0.9 +/- 0.3) x 10(-8) (benzothienylcephalosporin) to (10 +/- 3.3) x 10(-8) cm/s (cephaloridine); i.e., the P values were lower than those reported for Pseudomonas aeruginosa and Escherichia coli by 1 and 3 orders of magnitude, respectively. The permeability barrier was shown to reduce drastically the stream of drug molecules entering the cell, allowing the rather low level of beta-lactamase (0.1 U/mg of protein with penicillin G) to decrease radically the concentration of the drug at the target; this explains the poor in vitro activities of the beta-lactams against M. chelonei. We also estimated P for small, hydrophilic molecules (glucose, glycerol, glycine, leucine), by studying their uptake kinetics. The values found, ranging from 15 x 10(-8) to 490 x 10(-8) cm/s, were consistent again with a very low permeability of M. chelonei cell wall. The permeation of cephalosporins was not very dependent on the hydrophobicity of the molecules or on the temperature, suggesting a hydrophilic pathway of penetration for these molecules.

Glucose transport in Brucella abortus

Brucella abortus British strain 19 transported glucose with an apparent K(m) of 0.16 mM and an apparent V(max) of 250 nmol per min per mg of N. The only common glucose analogue transported was 2-deoxyglucose (2-DOG), with an apparent K(i) of 0.73 mM. Alpha- or beta-methyl glucosides and 3-O-methylglucose were not transported. Transport was linear for 70 to 90 s, depending on the concentration of substrate used. 2-Deoxyglucose was transported as the free sugar and was not further metabolized once inside the cell. There was no glucose phosphoenolpyruvate phosphotransferase system (PEP-PTS) present, and there were no inhibitors present in Brucella cell-free extract that inhibited the Escherichia coli glucose PEP-PTS. N-Ethylmaleimide (NEM) and p-chloromercuribenzoate (pCMB) completely inhibited transport of glucose and 2-DOG. Glutathione, dithiothreitol, and beta-mercaptoethanol reversed the effects of pCMB but not of NEM. A pH optimum of 7.2 and a temperature optimum of 37 to 45 C were observed for both K(m) and V(max). The glucose transport system appeared to be constitutive for glucose transport in cells grown on fructose, galactose, erythritol, or glucose. The electron transfer inhibitors carbonyl cyanide, m-chlorophenylhydrazone, NaN(3), 2,4-dinitrophenol, and KCN inhibited 2-DOG transport to a greater extent than did the metabolic energy inhibitors NaAsO(4), iodoacetate, KF, and 2-heptyl-4-hydroxyquinoline-N-oxide. Dicyclohexylcarbodiimide, an inhibitor of membrane-bound adenosine triphosphatases, inhibited transport by 100%.

Uptake and utilization of glutamic acid by Cryptococcus albidus

Cryptococcus albidus utilizes glutamate as a sole carbon source. The kinetics of uptake of this amino acid were studied. l-Glutamic acid was taken up by two saturable systems: a high affinity system with a Michaelis constant (K(m)) of 1.15 x 10(-5) M and a V(max) of 0.049 mumol per mg per h and a low affinity system with a K(m) of 2.5 x 10(-3) M and a V(max) of 3.61 mumol per mg per h. Both systems possessed characteristics of active transport which were dependent on temperature and pH and which required metabolic energy. Uptake was inhibited at 37 C but the temperature-sensitive step was reversible. Chemical fractionation of cells with 5% trichloroacetic acid showed that glutamic acid initially entered a soluble pool which decreased after 1 h as the amino acid was incorporated into the protein and nucleic acid fractions of the yeast. Some of the glutamate was completely oxidized and could be recovered as (14)CO(2). Therefore, the amino acid was also used as an energy source.

Resistance to D-cycloserine in the tubercle bacilli: mutation rate and transport of alanine in parental cells and drug-resistant mutants

A single transport system was found to accumulate l- and d-alanine, glycine and d-serine in Mycobacterium tuberculosis. The results of inhibition experiments suggested that the antibiotics d-cycloserine and O-carbamyl-d-serine were also transported by the alanine-glycine-d-serine system. A d-cycloserine-resistant permease-competent (d-CS(r)/perm(+)) mutant and a d-cycloserine-resistant permease-defective (d-CS(r)/perm(-)) mutant were isolated. The d-CS(r)/perm(-) mutant was not found to be more resistant to the drug than was the d-CS(r)/perm(+) mutant. The data were consistent with the conclusion that resistance to d-cycloserine in the tubercle bacilli is primarily due to mutations in the gene(s) controlling the enzyme d-alanyl-d-alanine synthetase. The mutation rate was calculated to be about 10(-10) mutations per bacterium per generation.

Comparative physiologial studies of the yeast and mycelial forms of Histoplasma capsulaum: uptake and incorporation of L-leucine

l-Leucine entered the cells of both morphological forms of Histoplasma capsulatum by a permease-like system at low external concentrations of substrate. However, at levels greater than 5 x 10(-5)m l-leucine, the amino acid entered the cells both through a simple diffusion-like process and the permease-like system. The rate of the amino acid diffusion into yeast and mycelial forms appeared to be the same, whereas the initial rate of accumulation through the permease-like system was 5 to 10 times faster in the mycelial phase than it was in the yeast phase. The Michaelis constants were 2.2 x 10(-5)m in yeast phase and 2 x 10(-5)m in mycelial phase cells. Transport of l-leucine at an external concentration of 10(-5)m showed all of the characteristics of a system of active transport, which was dependent on temperature and pH. Displacement or removal of the alpha-amino group, or modification of the alpha-carboxyl group abolished amino acid uptake. The process was competitively inhibited by neutral aliphatic side-chain amino acids (inhibition constants ranged from 1.5 x 10(-5) to 6.2 x 10(-5)m). Neutral aromatic side-chain amino acids and the d-isomers of leucine and valine did not inhibit l-leucine uptake. These data were interpreted to mean that the l-leucine transport system is stereospecific and is highly specific for neutral aliphatic side-chain amino acids. Incorporation of l-leucine into macromolecules occurred at almost the same rate in both morphological forms of the fungus. The mycelial phase but not the yeast phase showed a slight initial lag in incorporation. In both morphological forms the intracellular pool of l-leucine was of limited capacity, and the total uptake of the amino acid was a function of intracellular pool size. The initial rate of l-leucine uptake was independent of the level of intracellular pool. Both morphological forms deaminated and degraded only a minor fraction of the accumulated leucine.