Bacterial chemotaxis to saccharides is governed by a trade-off between sensing and uptake

To swim up gradients of nutrients, E. coli senses nutrient concentrations within its periplasm. For small nutrient molecules, periplasmic concentrations typically match extracellular concentrations. However, this is not necessarily the case for saccharides, such as maltose, which is transported into the periplasm via a specific porin. Previous observations have shown that under various conditions E. coli limits maltoporin abundance so that, for extracellular micromolar concentrations of maltose, there are predicted to be only nanomolar concentrations of free maltose in the periplasm. Thus, in the micromolar regime, the total uptake of maltose from the external environment into the cytoplasm is limited not by the abundance of cytoplasmic transport proteins but by the abundance of maltoporins. Here we present results from experiments and modeling showing that this porin-limited transport enables E. coli to sense micromolar gradients of maltose despite having a high-affinity ABC transport system that is saturated at these micromolar levels. We used microfluidic assays to study chemotaxis of E. coli in various gradients of maltose and methyl-aspartate and leveraged our experimental observations to develop a mechanistic transport-and-sensing chemotaxis model. Incorporating this model into agent-based simulations, we discover a trade-off between uptake and sensing: although high-affinity transport enables higher uptake rates at low nutrient concentrations, it severely limits dynamic sensing range. We thus propose that E. coli may limit periplasmic uptake to increase its chemotactic sensitivity, enabling it to use maltose as an environmental cue.

The Blockade of Glutamate N-methyl-D-aspartate Receptors into the Prelimbic of Prefrontal Cortex Decreases Morphine-induced Conditioned Place Preference in Rat

BACKGROUND: The prelimbic area (PL) of the prefrontal cortex is susceptible to abnormal developmental stimuli that raises the risk of addiction. Glutamate receptors play a key role in opiate reinforcement and reward functions in this area. Therefore, we examined the effect of the DL-2-amino-5-phosphonopentanoic acid (AP5), as N-methyl-D-aspartate (NMDA) receptor antagonist into the PL on the phases of conditioned place preference (CPP) induced by morphine.METHODS: Male Wistar rats were divided into 12 groups (3 surgical groups for each dose of morphine in any phase of CPP) and anaesthetized with chloral hydrate. Cannula was implanted into the PL and the AP5 was injected into this area and morphine-induced CPP was investigated. Data were processed with the commercially available SPSS 22 software using one-way ANOVA and Tukey's test. p<0.05 were considered statistically significant.RESULTS: Our findings indicated, morphine in doses of 2.5 to 10 mg/kg induced CPP. Microinjection of various doses of the AP5 into the PL before the administration of the effective dose of morphine significantly reduced place preference in the acquisition and the expression phases of the CPP test compared to the sham group (p<0.001). In another set of our experiments was seen that, different doses of the AP5 with the ineffective dose of morphine only reduced the expression phase of the CPP (p<0.001) while, produced neither preference nor aversion effect on the acquisition phase (p=0.147).CONCLUSION: It seems that the glutamate NMDA receptors in the PL through memory formation and morphine-related reward signals play a critical role in addiction process during morphine-induced CPP.KEYWORDS: N-methyl-aspartate, morphine, glutamate receptor, prefrontal cortex, reward

Characterization of the 2-Hydroxy-acid Dehydrogenase McyI, Encoded within the Microcystin Biosynthesis Gene Cluster of Microcystis aeruginosa PCC7806

The cyanobacterium Microcystis aeruginosa is widely known for its production of the potent hepatotoxin microcystin. This cyclic heptapeptide is synthesized non-ribosomally by the thio-template function of a large modular enzyme complex encoded within the 55-kb microcystin synthetase gene (mcy) cluster. The mcy gene cluster also encodes several stand-alone enzymes, putatively involved in the tailoring and export of microcystin. This study describes the characterization of the 2-hydroxy-acid dehydrogenase McyI, putatively involved in the production of d-methyl aspartate at position 3 within the microcystin cyclic structure. A combination of bioinformatics, molecular, and biochemical techniques was used to elucidate the structure, function, regulation, and evolution of this unique enzyme. The recombinant McyI enzyme was overexpressed in Escherichia coli and enzymatically characterized. The hypothesized native activity of McyI, the interconversion of 3-methyl malate to 3-methyl oxalacetate, was demonstrated using an in vitro spectrophotometric assay. The enzyme was also able to reduce α-ketoglutarate to 2-hydroxyglutarate and to catalyze the interconversion of malate and oxalacetate. Although NADP(H) was the preferred cofactor of the McyI-catalyzed reactions, NAD(H) could also be utilized, although rates of catalysis were significantly lower. The combined results of this study suggest that hepatotoxic cyanobacteria such as M. aeruginosa PCC7806 are capable of producing methyl aspartate via a novel glutamate mutase-independent pathway, in which McyI plays a pivotal role.