38 research outputs found
Chloroplasts in plant cells show active glassy behavior under low-light conditions
Plants have developed intricate mechanisms to adapt to changing light
conditions. Besides photo- and helio- tropism -- the differential growth
towards light and the diurnal motion with respect to sunlight -- chloroplast
motion acts as a fast mechanism to change the intracellular structure of leaf
cells. While chloroplasts move towards the sides of the plant cell to avoid
strong light, they accumulate and spread out into a layer on the bottom of the
cell at low light to increase the light absorption efficiency. Although the
motion of chloroplasts has been studied for over a century, the collective
organelle-motion leading to light adapting self-organized structures remains
elusive. Here we study the active motion of chloroplasts under dim light
conditions, leading to an accumulation in a densely packed quasi-2D layer. We
observe burst-like re-arrangements and show that these dynamics resemble
colloidal systems close to the glass transition by tracking individual
chloroplasts. Furthermore, we provide a minimal mathematical model to uncover
relevant system parameters controlling the stability of the dense configuration
of chloroplasts. Our study suggests that the meta-stable caging close to the
glass-transition in the chloroplast mono-layer serves a physiological
relevance. Chloroplasts remain in a spread-out configuration to increase the
light uptake, but can easily fluidize when the activity is increased to
efficiently re-arrange the structure towards an avoidance state. Our research
opens new questions about the role that dynamical phase transitions could play
in self-organized intracellular responses of plant cells towards environmental
cues
Billiards with Spatial Memory
Many classes of active matter develop spatial memory by encoding information
in space, leading to complex pattern formation. It has been proposed that
spatial memory can lead to more efficient navigation and collective behaviour
in biological systems and influence the fate of synthetic systems. This raises
important questions about the fundamental properties of dynamical systems with
spatial memory. We present a framework based on mathematical billiards in which
particles remember their past trajectories and react to them. Despite the
simplicity of its fundamental deterministic rules, such a system is strongly
non-ergodic and exhibits highly-intermittent statistics, manifesting in complex
pattern formation. We show how these self-memory-induced complexities emerge
from the temporal change of topology and the consequent chaos in the system. We
study the fundamental properties of these billiards and particularly the
long-time behaviour when the particles are self-trapped in an arrested state.
We exploit numerical simulations of several millions of particles to explore
pattern formation and the corresponding statistics in polygonal billiards of
different geometries. Our work illustrates how the dynamics of a single-body
system can dramatically change when particles feature spatial memory and
provide a scheme to further explore systems with complex memory kernels.Comment: 11 pages, 6 figure
Stress-Induced Dinoflagellate Bioluminescence at the Single Cell Level.
One of the characteristic features of many marine dinoflagellates is their bioluminescence, which lights up nighttime breaking waves or seawater sliced by a ship's prow. While the internal biochemistry of light production by these microorganisms is well established, the manner by which fluid shear or mechanical forces trigger bioluminescence is still poorly understood. We report controlled measurements of the relation between mechanical stress and light production at the single cell level, using high-speed imaging of micropipette-held cells of the marine dinoflagellate Pyrocystis lunula subjected to localized fluid flows or direct indentation. We find a viscoelastic response in which light intensity depends on both the amplitude and rate of deformation, consistent with the action of stretch-activated ion channels. A phenomenological model captures the experimental observations.Gordon and Betty Moore Foundation
Schlumberger Chair Fund
French government funding (ANR
Dissecting Bottromycin Biosynthesis Using Comparative Untargeted Metabolomics.
Bottromycin A2 is a structurally unique ribosomally synthesized and post-translationally modified peptide (RiPP) that possesses potent antibacterial activity towards multidrug-resistant bacteria. The structural novelty of bottromycin stems from its unprecedented macrocyclic amidine and rare β-methylated amino acid residues. The N-terminus of a precursor peptide (BtmD) is converted into bottromycin A2 by tailoring enzymes encoded in the btm gene cluster. However, little was known about key transformations in this pathway, including the unprecedented macrocyclization. To understand the pathway in detail, an untargeted metabolomic approach that harnesses mass spectral networking was used to assess the metabolomes of a series of pathway mutants. This analysis has yielded key information on the function of a variety of previously uncharacterized biosynthetic enzymes, including a YcaO domain protein and a partner protein that together catalyze the macrocyclization.This work was supported by a BBSRC studentship (W.J.K.C.), BBSRC grant BB/M003140/1 (A.W.T. and J.S-A), a Royal Society University Research Fellowship (A.W.T.), and by the BBSRC MET ISP grant to the John Innes Centre.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/anie.20160430
Discovery of a Family of Radical S-Adenosylmethionine Enzymes that Install a Novel Lysine-to-Tryptophan Crosslink
Nature has evolved a wide array of strategies for synthesizing bioactive secondary metabolites. One strategy involves ribosomal biosynthesis of a precursor peptide, which is then modified by one or more tailoring enzymes. The modifications encompass an assortment of cyclization reactions, and the responsible enzymes can often be seen in the biosynthetic gene clusters of the corresponding metabolites. Our lab recently discovered one such cyclic peptide, streptide, which is produced in a quorum sensing-controlled fashion by Streptococcus thermophilus. We showed that streptide contains an unprecedented carbon-carbon crosslink between the β-carbon of a lysine residue and the indole-C7 of a tryptophan side chain. We further demonstrated that the formation of this unusual bond is dependent on a radical S-adenosylmethionine (SAM) metalloenzyme.
Herein we show that this enzyme, which we call StrB, contains multiple [4Fe-4S] clusters. One of these clusters binds in the active site and reductively cleaves SAM to generate a 5'-deoxyadenosyl radical, which initiates the radical reaction. Using in vitro kinetic assays coupled with spectroscopic analyses, we establish that StrB installs the novel Lys-to-Trp crosslink in a single step, thus demonstrating a new route for peptide cyclization. On the basis of site-directed mutagenesis of key enzymatic residues and use of substrate analogs containing deuterated amino acids and natural or unnatural amino acid substitutions, we propose a plausible mechanism for this unusual transformation. We further expand the family of Lys-Trp crosslinking enzymes to AgaB and SuiB, StrB homologs encoded in pathogenic Streptococci. Lastly, we report the chemo-enzymatic total synthesis of streptide homologs from these pathogens, which will guide future functional studies of the new family of streptide secondary metabolites
Institutions, Transaction Costs and Entry Mode Decisions : The Case of Swedish SMEs in India
In the current third wave of internationalization companies from mature markets are investing in emerging markets and increase their foreign activities. For this internationalization process, companies need to enter the market with an appropriate entry mode strategy. Prior research focused mainly on MNC entry modes and also on factors as ownership, location or internalization advantages and not on SMEs and transaction cost theory. This thesis deals with the topic of institutions, transaction costs and entry mode decisions of Swedish SMEs in India. The purpose of this thesis is to understand the managerial perceptions about the influence of institutions on the degree of linkage specificity with which the transaction costs will be explored. This will lead to further knowledge about certain entry mode decisions of Swedish SMEs for the Indian market. This was studied with a qualitative research strategy using a multiple case study method. The empirical data was conducted via secondary data and primary data was collected via interviews with the sales responsible of the four case companies Norden Machinery AB, Slipnaxos AB, Håkansson Sågblad AB and Hedin Lagan AB. Main findings of the research were that institutions are perceived differently by the managers and therefore the institutions dissimilarly influenced the perceptions of the transaction costs. Furthermore, the study revealed that some managers decided their entry mode on the base of the perception of the transaction costs and some managers did not consider transaction costs when entering new markets. All in all it can be stated that transaction costs influence the entry mode of companies. The research is limited by the fact that transaction cost theory in general neglects factors as production costs. Managerial implications are that transactions costs should not be neglected as they help to choose a more successful entry mode and that the assets specificity and the behavioral uncertainties need to be taken into account when deciding upon an entry mode. Theory profits from this thesis as it proved that transaction costs influence entry mode decisions and that the linkage specificity is an important factor to include when combining transaction cost theory with entry mode decisions
Lysine-Tryptophan-Crosslinked Peptides Produced by Radical SAM Enzymes in Pathogenic Streptococci
Macrocycles represent
a common structural framework in many naturally occurring peptides.
Several strategies exist for macrocyclization, and the enzymes that
incorporate them are of great interest, as they enhance our repertoire
for creating complex molecules. We recently discovered a new peptide
cyclization reaction involving a crosslink between the side chains
of lysine and tryptophan that is installed by a radical SAM enzyme.
Herein, we characterize relatives of this metalloenzyme from the pathogens <i>Streptococcus agalactiae</i> and <i>Streptococcus suis</i>. Our results show that the corresponding enzymes, which we call
AgaB and SuiB, contain multiple [4Fe-4S] clusters and catalyze Lys-Trp
crosslink formation in their respective substrates. Subsequent high-resolution-MS
and 2D-NMR analyses located the site of macrocyclization. Moreover,
we report that AgaB can accept modified substrates containing natural
or unnatural amino acids. Aside from providing insights into the mechanism
of this unusual modification, the substrate promiscuity of AgaB may
be exploited to create diverse macrocyclic peptides