13 research outputs found
FtsZ-independent septal recruitment and function of cell wall remodelling enzymes in chlamydial pathogens.
The nature and assembly of the chlamydial division septum is poorly defined due to the paucity of a detectable peptidoglycan (PG)-based cell wall, the inhibition of constriction by penicillin and the presence of coding sequences for cell wall precursor and remodelling enzymes in the reduced chlamydial (pan-)genome. Here we show that the chlamydial amidase (AmiA) is active and remodels PG in Escherichia coli. Moreover, forward genetics using an E. coli amidase mutant as entry point reveals that the chlamydial LysM-domain protein NlpD is active in an E. coli reporter strain for PG endopeptidase activity (ÎnlpI). Immunolocalization unveils NlpD as the first septal (cell-wall-binding) protein in Chlamydiae and we show that its septal sequestration depends on prior cell wall synthesis. Since AmiA assembles into peripheral clusters, trimming of a PG-like polymer or precursors occurs throughout the chlamydial envelope, while NlpD targets PG-like peptide crosslinks at the chlamydial septum during constriction
Ants in a Labyrinth: A Statistical Mechanics Approach to the Division of Labour
Division of labour (DoL) is a fundamental organisational principle in human
societies, within virtual and robotic swarms and at all levels of biological
organisation. DoL reaches a pinnacle in the insect societies where the most
widely used model is based on variation in response thresholds among
individuals, and the assumption that individuals and stimuli are well-mixed.
Here, we present a spatially explicit model of DoL. Our model is inspired by
Pierre de Gennes' 'Ant in a Labyrinth' which laid the foundations
of an entire new field in statistical mechanics. We demonstrate the emergence,
even in a simplified one-dimensional model, of a spatial patterning of
individuals and a right-skewed activity distribution, both of which are
characteristics of division of labour in animal societies. We then show using a
two-dimensional model that the work done by an individual within an activity
bout is a sigmoidal function of its response threshold. Furthermore, there is an
inverse relationship between the overall stimulus level and the skewness of the
activity distribution. Therefore, the difference in the amount of work done by
two individuals with different thresholds increases as the overall stimulus
level decreases. Indeed, spatial fluctuations of task stimuli are minimised at
these low stimulus levels. Hence, the more unequally labour is divided amongst
individuals, the greater the ability of the colony to maintain homeostasis.
Finally, we show that the non-random spatial distribution of individuals within
biological and social systems could be caused by indirect (stigmergic)
interactions, rather than direct agent-to-agent interactions. Our model links
the principle of DoL with principles in the statistical mechanics and provides
testable hypotheses for future experiments
Regulatory (pan-)genome of an obligate intracellular pathogen in the PVC superphylum.
Like other obligate intracellular bacteria, the Chlamydiae feature a compact regulatory genome that remains uncharted owing to poor genetic tractability. Exploiting the reduced number of transcription factors (TFs) encoded in the chlamydial (pan-)genome as a model for TF control supporting the intracellular lifestyle, we determined the conserved landscape of TF specificities by ChIP-Seq (chromatin immunoprecipitation-sequencing) in the chlamydial pathogen Waddlia chondrophila. Among 10 conserved TFs, Euo emerged as a master TF targeting >100 promoters through conserved residues in a DNA excisionase-like winged helix-turn-helix-like (wHTH) fold. Minimal target (Euo) boxes were found in conserved developmentally-regulated genes governing vertical genome transmission (cytokinesis and DNA replication) and genome plasticity (transposases). Our ChIP-Seq analysis with intracellular bacteria not only reveals that global TF regulation is maintained in the reduced regulatory genomes of Chlamydiae, but also predicts that master TFs interpret genomic information in the obligate intracellular α-proteobacteria, including the rickettsiae, from which modern day mitochondria evolved
Functional dichotomy and distinct nanoscale assemblies of a cell cycle-controlled bipolar zinc-finger regulator
Dynamic Polyethism and Competition for Tasks in Threshold Reinforcement Models of Social Insects
Ătude de l'impact d'incendies de forĂȘt rĂ©pĂ©tĂ©s sur la biodiversitĂ© et sur les sols. Recherche d'indicateurs. Rapport final dĂ©taillĂ©.
Goals: This project aimed at assessing the impact of forest fire repetition on vegetation and the soil, in terms of biodiversity, carbon balance and the global ecosystem potential. The study area is the Maures massif (Mediterranean area in the South-East of France), an ecosystem developed on crystalline rocks. Because of the persistent drought during the project, we studied the interaction between fire recurrence and drought recurrence. Protocols: The sampling design includes 3 fire regimes during the last 50 years (many fires = 3-5, medium = 1-2 fires, control = no fire) and 3 time spans since the last fire (very recent = 9-12 months, recent = 3-5 years, old = 15-25 years). Control plots did not burn since either 50 years, or 200 years. 31 plots were laid out, five by treatment except for the old growth control forest (1 plot). The follow-up of the same plots during 3 years allowed furthermore studying the short-term dynamics of the environment after fire. We studied the functional relations between various aerial and soil compartments of the ecosystem at several spatial and temporal scales, combining physical, chemical and biologic factors, to identify key processes and species controlling the resilience of the environment or its resistance to disturbances. Results: Globally, the time since the last fire, thus the ecosystem resilience dynamics, were more influent on most of the measured parameters than the number or the frequency of fires. However, these last ones often play a functionally significant role, their relative importance increasing with the number of fires. The absence of fires for 150 - 200 years allowed a dramatic recovery of the fertility and biological activity. The repletion of the carbon stock in the superficial soil horizons (0 15 cm) and important modifications in flora structure and composition were particularly significant. With a mean fire frequency of one fire every 25 to 50 years, a good global resilience of the ecosystem, its flora and its functioning was observed, but at a low level of potential and at the shrinkage limit: most of the organic matter was limited to the first centimetres of the topsoil (0-5cm) where the biologic activity concentrated at fire's mercy. Most of the parameters which are modified by the fire recovered quantitatively in 15 to 25 years. But 50 years were generally necessary for the resilience of qualitative aspects, notably for the organic matter, biogeochemical cycles, microbiologic functioning of the soil and plant diversity.With higher fire frequencies (4 fires or more in 50 years, or fires occurring at less than 10 years interval), many parameters are durably altered, showing a global degradation of the ecosystem potential: it was particularly true for vegetation composition and structure, soil fauna and the loss of organic matter and its qualitative transformation. This last change led to drop in the activity, the resistance and the resilience of microbial communities. The nitrogen cycle was particularly disturbed. With one to 3 fires in 50 years, these forests do not store nor lose significantly carbon in the superficial soil horizons nor in the living biomass; a higher frequency led the forest to be a carbon source. With no fires over 50 years, and particularly after more than 150 years, these forests become a good carbon sink. The conjunction of numerous fires and repeated droughts led to a collapse of the biological functioning of the ecosystem. Fire and drought amplified mutually their deleterious impact. The persistent drought from 2003 to 2007 stopped the normally fast recovery dynamics in the first year after the fire of year 2003, and even made some biologic parameters decline. Plots with the higher fire frequency up to 2003 were the most impacted by drought. The preliminary effect of drought also dramatically increased the impact of the fires of year 2007. A threshold of 4 successive years of severe drought seems to be a critical threshold for the ecosystem, as well as 4 fires would be a critical threshold for the number of fires in 50 years. A set of indicators of the health status and dynamics of the ecosystem was selected among biological, physical and chemical parameters.Objectifs : Ce projet avait pour objectif d'Ă©valuer l'impact de la rĂ©pĂ©tition des feux de forĂȘt sur le sol et la vĂ©gĂ©tation en termes de biodiversitĂ©, de bilan de carbone et de dĂ©gradation gĂ©nĂ©rale des potentialitĂ©s de l'Ă©cosystĂšme. La zone d'Ă©tude se situe dans le massif des Maures (zone mĂ©diterranĂ©enne au Sud-Est de la France, Ă©cosystĂšme dĂ©veloppĂ©s sur roches cristallines. Nous avons simultanĂ©ment Ă©tudiĂ© l'interaction entre rĂ©currence des feux et rĂ©currence des sĂ©cheresses. Protocoles : Le plan d'Ă©chantillonnage comprend 3 rĂ©gimes d'incendies au cours des 50 derniĂšres annĂ©es (nombreux = 3-5 feux, peu nombreux = 1-2 feux, tĂ©moin) et 3 modalitĂ©s de date du dernier feu (trĂšs rĂ©cent = 9 mois -1 an, rĂ©cent =3 ans, ancien = 15-25 ans), les tĂ©moins n'ayant pas brĂ»lĂ© depuis soit 50 ans, soit 200 ans ou plus. Il y a 5 placettes par modalitĂ© sauf tĂ©moin trĂšs ancien, soit au total 31 placettes. Le suivi sur plusieurs annĂ©es des mĂȘmes placettes a permis de plus d'Ă©tudier la dynamique Ă court terme du milieu aprĂšs le feu. Nous avons Ă©tudiĂ© les relations fonctionnelles entre diffĂ©rents compartiments aĂ©riens et souterrains Ă plusieurs niveaux des Ă©chelles spatiale et temporelle de l'Ă©cosystĂšme, en combinant des facteurs physiques, chimiques et biologiques, afin d'identifier des processus et espĂšces clefs de la rĂ©silience du milieu ou de sa rĂ©sistance aux perturbations. RĂ©sultats : Globalement, le temps depuis le dernier feu, donc la dynamique de reconstitution de l'Ă©cosystĂšme, influence plus les paramĂštres mesurĂ©s que le nombre ou la frĂ©quence des feux, mais ces derniers jouent cependant un rĂŽle souvent significatif et fonctionnellement important, plus rarement dominant. L'absence de feux durant 150 Ă 200 ans permet une spectaculaire remontĂ©e biologique et du potentiel de l'Ă©cosystĂšme, marquĂ©e par la reconstitution d'un stock significatif de carbone dans les horizons superficiels du sol (0 Ă 15 cm) et une modification importante de la structure et composition floristique. Une frĂ©quence moyenne d'incendie de l'ordre d'un feu tous les 25 Ă 50 ans permet une bonne rĂ©silience globale de l'Ă©cosystĂšme, de sa flore et de son fonctionnement, mais Ă un niveau faible de potentialitĂ©s et en limite de la rupture. L'essentiel du stock de matiĂšre organique est limitĂ© aux premiers centimĂštres du sol (0-5cm) oĂč se concentre l'activitĂ© biologique. La plupart des paramĂštres qui sont modifiĂ©s par le feu rĂ©cupĂšrent quantitativement au bout de 15 Ă 25 ans. Mais il faut en gĂ©nĂ©ral 50 ans pour les aspects qualitatifs, notamment au niveau de la matiĂšre organique, des cycles biogĂ©ochimiques, du fonctionnement microbiologique du sol et de la biodiversitĂ© vĂ©gĂ©tale. Avec des frĂ©quences plus Ă©levĂ©es, aboutissant Ă 4 feux ou plus en 50 ans ou Ă quelques feux trĂšs rapprochĂ©s (5-10 ans), de nombreux paramĂštres chimiques et biologiques sont durablement altĂ©rĂ©s, et marquent une dĂ©gradation globale des potentialitĂ©s du systĂšme. On note un changement durable de la composition et de la structure de la vĂ©gĂ©tation et une perte significative de matiĂšre organique et sa transformation qualitative, responsables d'une baisse sensible de l'activitĂ©, de la rĂ©sistance et de la rĂ©silience microbiologique. Le cycle de l'azote est particuliĂšrement touchĂ©. Pour les fonctions de sĂ©questration de carbone : dans un rĂ©gime moyen d'incendie de l'ordre d'un feu tous les 25 Ă 50 ans, ces forĂȘts ne stockent pas ni ne perdent significativement de carbone dans les horizons superficiels du sol et dans la biomasse vivante ; elles en perdent par contre en cas de feux rĂ©pĂ©tĂ©s plus souvent. Elles possĂšdent un grand potentiel de stockage (effet puits de CO2) en cas ne non-brĂ»lage sur des durĂ©es de 150 Ă 200 ans. La conjonction de nombreux feux et de sĂ©cheresses rĂ©pĂ©tĂ©es conduit Ă un effondrement du fonctionnement biologique de l'Ă©cosystĂšme dans ses parties aĂ©rienne comme souterraine. Chacune des perturbations (feu et sĂ©cheresse) amplifie les effets nĂ©fastes de l'autre. La sĂ©cheresse persistante a non seulement totalement arrĂȘtĂ© la dynamique normalement rapide de l'Ă©cosystĂšme entre la 3Ăšme et la 5Ăšme annĂ©e aprĂšs le feu, mais a fait largement rĂ©gresser certains paramĂštres biologiques. Les placettes les plus impactĂ©es par le feu (incendies rĂ©cents ou nombreux) ont Ă©tĂ© les plus touchĂ©es. L'effet prĂ©alable de la sĂ©cheresse a aussi considĂ©rablement aggravĂ© l'impact d'un feu survenu en 2007. Un seuil de 4 annĂ©es successives de sĂ©cheresse marquĂ©e semble constituer un seuil critique pour l'Ă©cosystĂšme, de la mĂȘme maniĂšre que 4 feux seraient un seuil critique pour le nombre de feux en 50 ans. Le changement climatique ne peut donc que le fragiliser d'avantage ces Ă©cosystĂšmes, en accentuant les effets nĂ©fastes des incendies et en les multipliant. Un jeu d'indicateurs de l'Ă©tat et de l'Ă©volution de l'Ă©cosystĂšme a Ă©tĂ© retenu, basĂ© sur des paramĂštres biologiques, physiques et chimiques
Sécheresses et incendies répétés accroissent mutuellement leur impact sur l'écosystÚme
International audienceIn a recent research project performed in South-Eastern France on the impact of repeated fires on many components of forest environment (vegetation, fauna, microbiology, soil physics, chemical properties, organic matter, nutrients) we showed a strong interaction between repeated droughts and repeated fires. Each disturbance significantly increases the impact of the other one. Repeated droughts can stop and even reverse the recovery process after fire, and delay this recovery when they occurred before fire. Repeated fires lessen the resistance and resilience of the ecosystem to drought. Forest regeneration processes are threatened even in usually fire-prone environments usually resilient. Soil biological activity is severely affected, and particularly some key groups as earthworms and bacteria contributing to nitrogen cycle. Soil physics and chemical properties appear to be degraded as a habitat, and indirectly the reduction of biological activity limits their recovery, including a negative carbon balance. Four successive years appear to be the critical threshold for drought and four times in 50 years the critical threshold for fires. As climate change may lead to higher drought frequency and fire occurrence is tied to drought, the drought/fire interaction may degrade forest ecosystems more rapidly than expected from separate assessment of drought and fire impacts