Evidence for a monomeric structure of nonribosomal Peptide synthetases

Nonribosomal peptide synthetases (NRPS) are multimodular biocatalysts that bacteria and fungi use to assemble many complex peptides with broad biological activities. The same modular enzymatic assembly line principles are found in fatty acid synthases (FAS), polyketide synthases (PKS), and most recently in hybrid NRPS/PKS multienzymes. FAS as well as PKS are known to function as homodimeric enzyme complexes, raising the question of whether NRPS may also act as homodimers. To test this hypothesis, biophysical methods (size exclusion chromatography, analytical equilibrium ultracentrifugation, and chemical crosslinking) and biochemical methods (two-affinity-tag-system and complementation studies with enzymes being inactivated in different catalytic domains) were applied to NRPS subunits from the gramicidin S (GrsA-ATE), tyrocidine (TycB(1)-CAT and TycB(2-3)-AT.CATE), and enterobactin (EntF-CATTe) biosynthetic systems. These methods had revealed the dimeric structure of FAS and PKS previously, but all three NRPS systems investigated are functionally active as monomers.     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

Cages, baskets, ladders, and tubes: conformational studies of polyhedral oligomeric silsesquioxanes

The conformational flexibility of a series of cage, basket, ladder, and tube polyhedral oligomeric silsesquioxanes (POSS) has been examined using the Low Mode:Monte Carlo conformational search method in conjunction with the MM3/GBSA(CHCl3) surface. An ensemble of low energy structures was generated and used to explore the molecular shape and flexibility of each system. The results indicate that, except for the ladder molecule, the incompletely condensed systems that are studied are relatively rigid. Even in cases where the molecule is able to adopt numerous low energy conformations, the overall shape remains cage-like and the conformations differ only by small angles or substituent orientations. The ladder molecule is the most flexible and this ensemble clusters into two families: one that is cage-like and the other that is more open and ladder-like. The conformational flexibilities in the gas and solvent phases, as approximated using the GBSA continuum solvent model, are very similar.     

Yersiniabactin synthetase: a four-protein assembly line producing the nonribosomal peptide/polyketide hybrid siderophore of Yersinia pestis

Yersiniabactin synthetase comprises four proteins, YbtE, HMWP1, HMWP2, and YbtU, encompassing seventeen functional domains, twelve catalytic and five carrier, to select, activate, and incorporate salicylate, three cysteines, and one malonyl moiety into the iron chelator yersiniabactin (Ybt). In the present study, yersiniabactin has been reconstituted in vitro from the 4 protein assembly line by the use of eight biosynthetic precursors. The rate of one turnover, comprising 22 chemical operations performed by the assembly line to release the completed Ybt molecule, was determined at 1.4 min(-1). During the course of Ybt production, the elongating acyl-S-enzyme chain was shown to transfer across a nonribosomal peptide synthetase/polyketide synthase (NRPS/PKS) interprotein interface and then a PKS/NRPS intraprotein interface. This study on the Ybt synthetase assembly line represents the first complete in vitro reconstitution of a nonribosomal peptide/polyketide hybrid system.     

Rapid catalytic oxidation of CO to CO(2) - On the development of a new approach to on-line oxygen isotope analysis of organic matter

A new approach to on-line oxygen isotope analysis has been developed which utilises existing elemental analyser and mass spectrometry technology to produce a sample of carbon dioxide gas for oxygen isotople analysis. The method relies on on-line high temperature pyrolysis of the sample over a carbon source followed by a rapid, non-contributive partial catalytic oxidation over nickel powder at between 550 and 600 degrees C. Initial results demonstrate both good precision (better than 0.2 per thousand) and accuracy for both cellulose and silver nitrate samples. Copyright 1999 John Wiley & Sons, Ltd.     

Editorial for Focus Issue on Enabling Science and Technology for High-Speed Optical Communications

<正>Fiber optics underpins the communications infrastructure for the information society today. Cables consisting of hair-like fibers speed data around the globe in the form of rapid pulses of light. Modern telecom