Glucose transport into the extremely halophilic archaebacteria

A conserved transcription factor controls gluconeogenesis via distinct targets in hypersaline-adapted archaea with diverse metabolic capabilities

Timely regulation of carbon metabolic pathways is essential for cellular processes and to prevent futile cycling of intracellular metabolites. In Halobacterium salinarum, a hypersaline adapted archaeon, a sugar-sensing TrmB family protein controls gluconeogenesis and other biosynthetic pathways. Notably, Hbt. salinarum does not utilize carbohydrates for energy, uncommon among Haloarchaea. We characterized a TrmB-family transcriptional regulator in a saccharolytic generalist, Haloarcula hispanica, to investigate whether the targets and function of TrmB, or its regulon, is conserved in related species with distinct metabolic capabilities. In Har. hispanica, TrmB binds to 15 sites in the genome and induces the expression of genes primarily involved in gluconeogenesis and tryptophan biosynthesis. An important regulatory control point in Hbt. salinarum, activation of ppsA and repression of pykA, is absent in Har. hispanica. Contrary to its role in Hbt. salinarum and saccharolytic hyperthermophiles, TrmB does not act as a global regulator: it does not directly repress the expression of glycolytic enzymes, peripheral pathways such as cofactor biosynthesis, or catabolism of other carbon sources in Har. hispanica. Cumulatively, these findings suggest rewiring of the TrmB regulon alongside metabolic network evolution in Haloarchaea.

Dynamic Metabolite Profiling in an Archaeon Connects Transcriptional Regulation to Metabolic Consequences

Previous work demonstrated that the TrmB transcription factor is responsible for regulating the expression of many enzyme-coding genes in the hypersaline-adapted archaeon Halobacterium salinarum via a direct interaction with a cis-regulatory sequence in their promoters. This interaction is abolished in the presence of glucose. Although much is known about the effects of TrmB at the transcriptional level, it remains unclear whether and to what extent changes in mRNA levels directly affect metabolite levels. In order to address this question, here we performed a high-resolution metabolite profiling time course during a change in nutrients using a combination of targeted and untargeted methods in wild-type and ΔtrmB strain backgrounds. We found that TrmB-mediated transcriptional changes resulted in widespread and significant changes to metabolite levels across the metabolic network. Additionally, the pattern of growth complementation using various purines suggests that the mis-regulation of gluconeogenesis in the ΔtrmB mutant strain in the absence of glucose results in low phosphoribosylpyrophosphate (PRPP) levels. We confirmed these low PRPP levels using a quantitative mass spectrometric technique and found that they are associated with a metabolic block in de novo purine synthesis, which is partially responsible for the growth defect of the ΔtrmB mutant strain in the absence of glucose. In conclusion, we show how transcriptional regulation of metabolism affects metabolite levels and ultimately, phenotypes.

Archaeal S-layer glycoproteins: post-translational modification in the face of extremes

Corresponding to the sole or basic component of the surface (S)-layer surrounding the archaeal cell in most known cases, S-layer glycoproteins are in direct contact with the harsh environments that characterize niches where Archaea can thrive. Accordingly, early work examining archaeal S-layer glycoproteins focused on identifying those properties that allow members of this group of proteins to maintain their structural integrity in the face of extremes of temperature, pH, and salinity, as well as other physical challenges. However, with expansion of the list of archaeal strains serving as model systems, as well as growth in the number of molecular tools available for the manipulation of these strains, studies on archaeal S-layer glycoproteins are currently more likely to consider the various post-translational modifications these polypeptides undergo. For instance, archaeal S-layer glycoproteins can undergo proteolytic cleavage, both N- and O-glycosylation, lipid-modification and oligomerization. In this mini-review, recent findings related to the post-translational modification of archaeal S-layer glycoproteins are considered.

Glucose inhibits the formation of gas vesicles in Haloferax volcanii transformants

The effect of glucose on the formation of gas vesicles was investigated in Haloferax mediterranei and Hfx.volcanii transformants containing the mc-gvp gene cluster of Hfx. mediterranei (mc-vac transformants). Increasing amounts of glucose in the medium resulted in a successive decrease in the amount of gas vesicles in both species, with a complete inhibition of their formation at glucose concentrations of > 70 mM in mc-vac transformants, and 100 mM in Hfx. mediterranei. Maltose and sucrose imposed a similar inhibitory effect, whereas xylose, arabinose, lactose, pyruvate and 2-deoxy-glucose had no influence on the gas vesicle formation in mc-vac transformants. The activities of the two mc-vac promoters were strongly reduced in mc-vac transformants grown in the presence of > 50 mM glucose. The gas vesicle overproducing Delta D transformant (lacking the repressing protein GvpD) also showed a glucose-induced lack of gas vesicles, indicating that GvpD is not involved in the repression. The addition of glucose was useful to block gas vesicle formation at a certain stage during growth, and vice versa, gas vesicle synthesis could be induced when a glucose-grown culture was shifted to medium lacking glucose. Both procedures will enable the investigation of defined stages during gas vesicle formation.

Nutrient uptake by microorganisms according to kinetic parameters from theory as related to cytoarchitecture

The abilities of organisms to sequester substrate are described by the two kinetic constants specific affinity, a degrees, and maximal velocity Vmax. Specific affinity is derived from the frequency of substrate-molecule collisions with permease sites on the cell surface at subsaturating concentrations of substrates. Vmax is derived from the number of permeases and the effective residence time, tau, of the transported molecule on the permease. The results may be analyzed with affinity plots (v/S versus v, where v is the rate of substrate uptake), which extrapolate to the specific affinity and are usually concave up. A third derived parameter, the affinity constant KA, is similar to KM but is compared to the specific affinity rather than Vmax and is defined as the concentration of substrate necessary to reduce the specific affinity by half. It can be determined in the absence of a maximal velocity measurement and is equal to the Michaelis constant for a system with hyperbolic kinetics. Both are taken as a measure of tau, with departure of KM from KA being affected by permease/enzyme ratios. Compilation of kinetic data indicates a 10(8)-fold range in specific affinities and a smaller (10(3)-fold) range in Vmax values. Data suggest that both specific affinities and maximal velocities can be underestimated by protocols which interrupt nutrient flow prior to kinetic analysis. A previously reported inverse relationship between specific affinity and saturation constants was confirmed. Comparisons of affinities with ambient concentrations of substrates indicated that only the largest a degreesS values are compatible with growth in natural systems.

High-affinity maltose/trehalose transport system in the hyperthermophilic archaeon Thermococcus litoralis

The hyperthermophilic marine archaeon Thermococcus litoralis exhibits high-affinity transport activity for maltose and trehalose at 85 degrees C. The K(m) for maltose transport was 22 nM, and that for trehalose was 17 nM. In cells that had been grown on peptone plus yeast extract, the Vmax for maltose uptake ranged from 3.2 to 7.5 nmol/min/mg of protein in different cell cultures. Cells grown in peptone without yeast extract did not show significant maltose or trehalose uptake. We found that the compound in yeast extract responsible for the induction of the maltose and trehalose transport system was trehalose. [14C]maltose uptake at 100 nM was not significantly inhibited by glucose, sucrose, or maltotriose at a 100 microM concentration but was completely inhibited by trehalose and maltose. The inhibitor constant, Ki, of trehalose for inhibiting maltose uptake was 21 nM. In contrast, the ability of maltose to inhibit the uptake of trehalose was not equally strong. With 20 nM [14C]trehalose as the substrate, a 10-fold excess of maltose was necessary to inhibit uptake to 50%. However, full inhibition was observed at 2 microM maltose. The detergent-solubilized membranes of trehalose-induced cells contained a high-affinity binding protein for maltose and trehalose, with an M(r) of 48,000, that exhibited the same substrate specificity as the transport system found in whole cells. We conclude that maltose and trehalose are transported by the same high-affinity membrane-associated system. This represents the first report on sugar transport in any hyperthermophilic archaeon.

Evolution of carbohydrate metabolic pathways

Current studies of hyperthermophilic archaea and bacteria, the phylogenetically deepest-rooted and slowest-evolving extant organisms known, are allowing new insights into the nature of presumably ancient metabolic pathways. The apparent common occurrence of modified non-phosphorylated Entner-Doudoroff (ED) pathways among saccharolytic archaea and the absence of the conventional Embden-Meyerhof-Parnas (EMP) mode of glycolysis indicate that the ED pathway is the older route of carbohydrate dissimilation. However, gluconeogenesis via the "reversed" EMP route has been found in archaea. Thus, the EMP pathway was probably an anabolic pathway to begin with; its catabolic role came later, with the evolution of fructose phosphate kinases, using ATP, ADP or pyrophosphate as phosphate donors. Similarly, the presence of reductive reactions of the citric acid cycle in anaerobic archaea and the most deeply rooted bacteria, including autotrophs, indicates that the citric acid cycle was originally a reductive biosynthetic pathway.

Characterisation of glucose transport in the hyperthermophilic Archaeon Sulfolobus solfataricus

Sulfolobus solfiataricus is a hyperthermophilic Archaeon growing at 80 degrees C, pH 3. The glucose transport system of this organism has been characterised kinetically at this temperature and pH using 2-deoxy-D-glucose: the sugar analogue is transported into the cells with a Km = 1.8 +/- 0.3 microM and a Vmax = 3.6 +/- 0.1 nmol min(-1) (mg protein)(-1), with an intracellular accumulation of up to 200-fold over the extracellular concentration. Transport was significantly reduced at pH 5. Inhibition of 2-deoxy-D-glucose transport was investigated using a variety of sugars and sugar analogues; D-glucose, D-galactose and D-mannose showed the highest affinity for the transporter, with D-glucose possessing a Ki = 120 +/- 20 nM.