Effects of litter on seedling establishment: an indoor experiment with short-lived Brassicaceae species

Recruitment by seeds is essential both in vegetation dynamics and in supporting biodiversity in grasslands. The recruitment by seeds is feasible in suitable vegetation gaps from the seed rain and/or by establishment from persistent soil seed banks. Cessation of grassland management results in litter accumulation, which leads to the decline of species diversity by the decreased availability of open patches. Low amounts of litter is often beneficial, while high amounts of litter is detrimental for seed germination and seedling establishment of short-lived species. In a designed indoor experiment, we explored the effect of litter on seedling establishment by germinating six short-lived Brassicaceae species with both increasing seed mass and litter cover. We found that both seed mass and litter had significant effect on germination and establishment of the sown species. Small-seeded species were significantly negatively affected by the 300 and/or 600 g/m² litter layers. No negative litter effect was detected for species with high seed masses (Lepidium spp.). No overall significant positive litter effect was found, although for most of the species cumulative seedling numbers were not the highest at the bare soil pots. Our results suggest that the negative effects of litter are less feasible on the large-seeded short-lived species than on that of small-seeded ones.     

Characterization of Mn(II) and Mn(III) binding capability of natural siderophores desferrioxamine B and desferricoprogen as well as model hydroxamic acids

Complexes formed between Mn(II) ion and acetohydroxamic acid (HAha), benzohydroxamic acid (HBha), N-methyl-acetohydroxamic acid (HMeAha), DFB model dihydroxamic acids (H₂(3,4-DIHA), H₂(3,3-DIHA), H₂(2,5-DIHA), H₂(2,5-H,H-DIHA), H₂(2,4-DIHA), H₂(2,3-DIHA)) and two trihydroxamate based natural siderophores, desferrioxamine B (H₄DFB) and desferricoprogen (H₃DFC) have been investigated under anaerobic condition (and some of them also under aerobic condition). The pH-potentiometric results showed the formation of well-defined complexes with moderate stability. Monohydroxamic acids not, but all of the dihydroxamic acids and trihydroxamic acids were able to hinder the hydrolysis of the metal ion up to pH ca. 11. Maximum three hydroxamates were found to coordinate to the Mn(II) ion, but presence of water molecule in the inner-sphere was also indicated by the corresponding relaxivity values even in the tris-chelated complexes. Moreover, prototropic exchange processes were found to increase the relaxation rate of the solvent water proton over the value of [Mn_aqua]²⁺ in the protonated Mn(II)-siderophore complexes at physiological pH. The much higher stability of Mn(III)-hydroxamate (especially tris-chelated) complexes compared to the corresponding Mn(II)-containing species results in a significantly decreased formal potential compared to the Mn(III)_aqua/Mn(II)_aqua system. As a result, air oxygen becomes an oxidizing agent for these manganese(II)-hydroxamate complexes above pH 7.5. The oxidation processes, followed by UV-Vis spectrophotometry, were found to be stoichiometric only in the case of the tris-chelated complexes of siderophores, which predominate above pH 9. ESI-MS provided support about the stoichiometry and cyclic-voltammetry was used to determine the stability constants for the tris-chelated complexes, [Mn(HDFB)]⁺ and [MnDFC].     

Interaction of folic acid and some matrix metalloproteinase (MMP) inhibitor folate-γ-hydroxamate derivatives with Zn(II) and human serum albumin

Human serum albumin binding of folic acid and its γ-hydroxamate/carboxylate derivatives was studied by ultrafiltration and spectrofluorimetry, and it was found that the ligands exhibit a moderate binding (K_D ~ 2-50 μM), and the folate-γ-phenylalanine represents the highest conditional binding constant towards albumin. This feature may have importance in the serum transport processes of these ligands. Interaction of folic acid and its derivatives with Zn(II) was investigated in aqueous solution to obtain the composition and stabilities of the complexes by the means of pH-potentiometry, ¹H NMR and electrospray ionization mass spectrometry, together with the characterization of the proton dissociation processes and the hydro-lipophilic properties of the ligands. The formation of mono-ligand complexes was demonstrated in all cases and the contribution of the glutamyl carboxylates to the coordination was excluded. Binding of folic acid and its γ-carboxylate derivatives to Zn(II) via the pteridine moiety is suggested, while the (O,O) coordination fashion of the folate-γ-hydroxamate ligands has importance in their inhibitory activity against Zn(II)-containing matrix metalloproteinases. It was found that the enzyme inhibition of these folate-γ-hydroxamate ligands is mainly tuned by other features, such as the lipophilic character rather than the Zn(II)-chelate stability.     

Switches and latches: a biochemical tug-of-war between the kinases and phosphatases that control mitosis

Activation of the cyclin-dependent kinase (Cdk1) cyclin B (CycB) complex (Cdk1:CycB) in mitosis brings about a remarkable extent of protein phosphorylation. Cdk1:CycB activation is switch-like, controlled by two auto-amplification loops--Cdk1:CycB activates its activating phosphatase, Cdc25, and inhibits its inhibiting kinase, Wee1. Recent experimental evidence suggests that parallel to Cdk1:CycB activation during mitosis, there is inhibition of its counteracting phosphatase activity. We argue that the downregulation of the phosphatase is not just a simple latch that suppresses futile cycles of phosphorylation/dephosphorylation during mitosis. Instead, we propose that phosphatase regulation creates coherent feed-forward loops and adds extra amplification loops to the Cdk1:CycB regulatory network, thus forming an integral part of the mitotic switch. These network motifs further strengthen the bistable characteristic of the mitotic switch, which is based on the antagonistic interaction of two groups of proteins: M-phase promoting factors (Cdk1:CycB, Cdc25, Greatwall and Endosulfine/Arpp19) and interphase promoting factors (Wee1, PP2A-B55 and a Greatwall counteracting phosphatase, probably PP1). The bistable character of the switch implies the existence of a CycB threshold for entry into mitosis. The end of G2 phase is determined by the point where CycB level crosses the CycB threshold for Cdk1 activation.     

Synthesis of N-glycopeptides applying glycoamino Acid building blocks with a combined fmoc/boc strategy

Mono-, di- and trisaccharide representing the reducing terminal of the core structure of N-glycans were incorporated into Leu-Lys-Asn-Gly-Gly-Pro hexapeptide that is a partial structure of the Trp-cage mini-protein by linear assembly. These studies provide evidence that the used combination of Fmoc and Boc strategy and mild conditions result in glycopeptides in high purity and reasonable yield.     

Cooperativity network of Trp‐cage miniproteins: probing salt‐bridges

Trp‐cage miniprotein was used to investigate the role of a salt‐bridge (Asp⁹–Arg¹⁶) in protein formation, by mutating residues at both sides, we mapped its contribution to overall stability and its role in folding mechanism. We found that both of the above side‐chains are also part of a dense interaction network composed of electrostatic, H‐bonding, hydrophobic, etc. components. To elucidate the fold stabilizing effects, we compared and contrasted electronic circular dichroism and NMR data of miniproteins equipped with a salt‐bridge with those of the salt‐bridge deleted mutants. Data were acquired both in neutral and in acidic aqueous solutions to decipher the pH dependency of both fully and partially charged partners. Our results indicate that the folding of Trp‐cage miniproteins is more complex than a simple two‐state process as we detected an intermediate state that differs significantly from the native fold. The intermediate formation is related to the salt‐bridge stabilization; in the miniprotein variants equipped with salt‐bridge the population of the intermediate state at acidic pH is significantly higher than it is for the salt‐bridge deleted mutants. In this molecular framework Arg¹⁶ stabilizes more than Asp⁹ does, because of its higher degree of 3D‐fold cooperation. In conclusion, the Xxx$^{9} \leftrightarrow$ Yyy¹⁶ salt‐bridge is not an isolated entity of this fold; rather it is an integrated part of a complex interaction network. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Detection of Staphylococcus aureus nasal carriage in healthy young adults from a Hungarian University

Asymptomatic carriage of Staphylococcus aureus in healthy individuals has a high prevalence, especially in children and young adults. Nasal colonisation is a well-known risk factor for subsequent severe infection, or can be the source of transmission of this bacterium to other susceptible persons. In this study, we have surveyed the nasal carriage rate of students of the Semmelweis University, by screening 300 volunteers. We have determined the antibiotic sensitivity of the isolates by Etest, and their genetic relatedness by pulsed-fieled gel electrophoresis. The nasal carriage rate of S. aureus was found to be 29.3%, and that of MRSA only 0.67% (2/300). The isolates were generally sensitive to antibiotics, except for macrolides. We could observe a noticeably great genetic diversity, even among strains deriving from students of the same university group.     

Protein phosphatase 2A controls the order and dynamics of cell-cycle transitions

Bistability of the Cdk1-Wee1-Cdc25 mitotic control network underlies the switch-like transitions between interphase and mitosis. Here, we show by mathematical modeling and experiments in Xenopus egg extracts that protein phosphatase 2A (PP2A), which can dephosphorylate Cdk1 substrates, is essential for this bistability. PP2A inhibition in early interphase abolishes the switch-like response of the system to Cdk1 activity, promoting mitotic onset even with very low levels of Cyclin, Cdk1, and Cdc25, while simultaneously inhibiting DNA replication. Furthermore, even if replication has already initiated, it cannot continue in mitosis. Exclusivity of S and M phases does not depend on bistability only, since partial PP2A inhibition prevents replication without inducing mitotic onset. In these conditions, interphase-level mitotic kinases inhibit Cyclin E-Cdk2 chromatin loading, blocking initiation complex formation. Therefore, by counteracting both Cdk1 activation and activity of mitotic kinases, PP2A ensures robust separation of S phase and mitosis and dynamic transitions between the two states.     

Chemotaxis induced by SXWS tetrapeptides in Tetrahymena--overlapping chemotactic effects of SXWS sequences and their identical amino acids

The chemotactic potential of SXWS peptides and the components of the extracellular domain of cytokine receptors were investigated in Tetrahymena as a functional index of substitution with different amino acids in the position 'X' of the tetrapeptide. Data obtained demonstrate that position X plays a special determining role in the ligand, SEWS and STWS possess extremely strong chemoattractant ability, and aromatic amino acids result in chemorepellent ligands. Diverse effects of structurally related molecules, for example, SNWS-SDWS, demonstrate a highly sensitive discrimination potential in the applied model system. Physicochemical characteristics (hydropathy, residue size, and solvent-exposed area) of the amino acids were correlated with the chemotactic activity. Data obtained by computer-assisted conformation analysis of SXWS peptides and the highly overlapping chemotactic effects of the investigated SXWS peptides as well as the presence of the amino acids in the 'X' position indicate that member 'X' of the SXWS sequence performs a special role in interactions with the chemotaxis receptors in the membrane.