3,414 research outputs found
Photoinduced Doughnut-Shaped Nanostructures
We show that an incoherent unpolarized single-beam illumination is able to
photoinduce nano-doughnuts on the surface of azopolymer thin films. We
demonstrate that individual doughnut-shaped nano-objects as well as clusters of
several adjacent nano-doughnuts can be formed and tailored with wide range of
typical sizes, thus providing a rich field for applications in nanophotonics
and photochemistry.Comment: 13 pages, 3 figures, first version to chem. phys. lett. 201
Comprehensive analysis of peptidoglycan hydrolases in Caulobacter crescentus
The peptidoglycan (PG) sacculus is a large macromolecule enclosing most bacterial cells. During progression of the cell cycle, it needs to be continuously remodelled to enable elongation of the cell body and, finally, cell division. This process requires a delicate balance between synthetic and hydrolytic reactions, which are executed by an array of different enzymes. However, the roles of individual components in this complex machinery and the mechanisms underlying their temporal and spatial regulation are still incompletely understood. In particular, the functional significance of many PG hydrolases is still unclear.
While acting at all stages of cell wall biogenesis, PG hydrolases have a particularly important role during cell division, facilitating the coordinated invagination of the PG layer as constriction proceeds. Previous work has identified the putative PG hydrolase DipM as a key component of the Caulobacter crescentus divisome. However, the extent to which other hydrolases contribute to PG remodelling during the constriction process in this species has remained unknown. To identify the major players and elucidate their function, I have analysed the full set of putative amidases, lytic transglycosylases (LTs), endopeptidases (EPases) and LD-transpeptidases (LD-TPases) encoded in the C. crescentus genome. For this purpose, I generated a variety of fluorescent fusions and deletion mutants and characterized them using microscopic and biochemical approaches.
The results obtained indicate that the PG hydrolases of C. crescentus have highly redundant functions. Based on the observation of changes in cell morphology, localization dynamics, stress and antibiotic resistance I have identified a particularly important role of EPases in maintaining cell wall integrity. Deletion of the amiC and chap genes encoding proteins with PG amidase activity had no detectable effect. In contrast, mutants lacking multiple EPases of the NlpC/P60 or LytM subgroups formed either long smooth filaments or occasionally cell chains. Furthermore, the presence of either NlpC/P60 domain containing EPase NlpA, or the Chap amidase is essential to maintain proper growth. Inactivation of all soluble lytic transglycosylases (SLTs), led to cell filamentation accompanied by outer-membrane blebbing. Depletion of DipM, an EPase homologue lacking catalytic activity in strains lacking either all SLTs or all of the remaining LytM factors (a subgroup of EPases), led to a complete block in cell division and finally to cell death. Interestingly, a similar morphological defect was observed upon depletion of a DipM in a strain lacking another LytM factor with degenerated active site, LdpF. Cultivation in stress conditions revealed critical functions of LdpF, SdpA (falling into the SLTs group) and Chap in cell envelope biogenesis, since single deletions of the respective genes led to either osmo- or antibiotic sensitivity. Moreover, SdpA displayed a dynamic localization pattern, characteristic for the divisome components implicated in the final stages of cell wall remodelling, indication a role in the cell division.
Collectively, this work provides the first comprehensive analysis of PG hydrolases in C. crescentus and underscores the key role of the catalytically inactive EPase homologues DipM and LdpF in the autolytic system of this species
La praxeÌologie comme grammaire chez Tadeusz Kotarbinski
La trajectoire philosophique de Tadeusz Kotarbinski se caracteÌrise avant tout par un caracteÌre complet de ses recherches. InteÌresseÌ deÌs le deÌbut de son travail aux questions dâeÌthique, il jugea neÌcessaire de se consacrer dâabord aux probleÌmes du langage et de lâaction. Il fut alors convaincu quâune bonne theÌorie de lâaction constitue un point de deÌpart neÌcessaire pour toute philosophie future â sans comprendre lâaction, nous ne pouvons pas comprendre ce qui en deÌcoule. Le preÌsent article tente dâesquisser les difficulteÌs de cette theÌorie que Kotarbinski a qualifieÌ de « praxeÌologie » et de comparer la deÌmarche de lâauteur polonais au travail dâun autre praxeÌologue important, Ludwig von Mises.Tadeusz Kotarbinskiâs philosophical development is astonishingly complete. From the very beginning of his work he showed interest in ethical questions, but he felt that before ha can answer them he needed to deal with several issues in the philosophy of language and action. He thought that a good theory of action (praxiology) constituted a necessary starting point for any future philosophy â without understanding action, we cannot understand what comes out of it. This article tries to sketch a few difficulties of this theory and to compare Kotarbinskiâs conception to the one proposed by another important praxiologist, Ludwig von Mises
La prĂ©caution Ă lâĂ©gard des principes
Lâexpression « principe de prĂ©caution » fonctionne souvent comme une maxime de prĂ©caution, et non pas comme un principe universel dâaction. Cet usage nâest toutefois pas celui qui est entendu par les personnes traitant de cette question dans le cadre des interrogations liĂ©es Ă la protection de lâenvironnement. Le principe de prĂ©caution, tel quâil a Ă©tĂ© inscrit dans la Charte de lâenvironnement, aspire Ă saisir quelque chose dâimportant et de nouveau dans le paysage scientifique français, et Ă donner un outil pour gĂ©rer cette nouveautĂ©. Lâarticle qui suit tente Ă montrer que cette aspiration nâest pas accomplie avant tout pour des raisons formelles : lâidĂ©e de prĂ©caution ne peut pas ĂȘtre traduite en un principe constitutionnel
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