Fluctuations in spo0A Transcription Control Rare Developmental Transitions in Bacillus subtilis

Phosphorylated Spo0A is a master regulator of stationary phase development in the model bacterium Bacillus subtilis, controlling the formation of spores, biofilms, and cells competent for transformation. We have monitored the rate of transcription of the spo0A gene during growth in sporulation medium using promoter fusions to firefly luciferase. This rate increases sharply during transient diauxie-like pauses in growth rate and then declines as growth resumes. In contrast, the rate of transcription of an rRNA gene decreases and increases in parallel with the growth rate, as expected for stable RNA synthesis. The growth pause-dependent bursts of spo0A transcription, which reflect the activity of the spo0A vegetative promoter, are largely independent of all known regulators of spo0A transcription. Evidence is offered in support of a “passive regulation” model in which RNA polymerase stops transcribing rRNA genes during growth pauses, thus becoming available for the transcription of spo0A. We show that the bursts are followed by the production of phosphorylated Spo0A, and we propose that they represent initial responses to stress that bring the average cell closer to the thresholds for transition to bimodally expressed developmental responses. Measurement of the numbers of cells expressing a competence marker before and after the bursts supports this hypothesis. In the absence of ppGpp, the increase in spo0A transcription that accompanies the entrance to stationary phase is delayed and sporulation is markedly diminished. In spite of this, our data contradicts the hypothesis that sporulation is initiated when a ppGpp-induced depression of the GTP pool relieves repression by CodY. We suggest that, while the programmed induction of sporulation that occurs in stationary phase is apparently provoked by increased flux through the phosphorelay, bet-hedging stochastic transitions to at least competence are induced by bursts in transcription

Built Shallow to Maintain Homeostasis and Persistent Infection: Insight into the Transcriptional Regulatory Network of the Gastric Human Pathogen Helicobacter pylori

Transcriptional regulatory networks (TRNs) transduce environmental signals into coordinated output expression of the genome. Accordingly, they are central for the adaptation of bacteria to their living environments and in host–pathogen interactions. Few attempts have been made to describe a TRN for a human pathogen, because even in model organisms, such as Escherichia coli, the analysis is hindered by the large number of transcription factors involved. In light of the paucity of regulators, the gastric human pathogen Helicobacter pylori represents a very appealing system for understanding how bacterial TRNs are wired up to support infection in the host. Herein, we review and analyze the available molecular and “-omic” data in a coherent ensemble, including protein–DNA and protein–protein interactions relevant for transcriptional control of pathogenic responses. The analysis covers ∼80% of the annotated H. pylori regulators, and provides to our knowledge the first in-depth description of a TRN for an important pathogen. The emerging picture indicates a shallow TRN, made of four main modules (origons) that process the physiological responses needed to colonize the gastric niche. Specific network motifs confer distinct transcriptional response dynamics to the TRN, while long regulatory cascades are absent. Rather than having a plethora of specialized regulators, the TRN of H. pylori appears to transduce separate environmental inputs by using different combinations of a small set of regulators

Reversible and Noisy Progression towards a Commitment Point Enables Adaptable and Reliable Cellular Decision-Making

Cells must make reliable decisions under fluctuating extracellular conditions, but also be flexible enough to adapt to such changes. How cells reconcile these seemingly contradictory requirements through the dynamics of cellular decision-making is poorly understood. To study this issue we quantitatively measured gene expression and protein localization in single cells of the model organism Bacillus subtilis during the progression to spore formation. We found that sporulation proceeded through noisy and reversible steps towards an irreversible, all-or-none commitment point. Specifically, we observed cell-autonomous and spontaneous bursts of gene expression and transient protein localization events during sporulation. Based on these measurements we developed mathematical population models to investigate how the degree of reversibility affects cellular decision-making. In particular, we evaluated the effect of reversibility on the 1) reliability in the progression to sporulation, and 2) adaptability under changing extracellular stress conditions. Results show that reversible progression allows cells to remain responsive to long-term environmental fluctuations. In contrast, the irreversible commitment point supports reliable execution of cell fate choice that is robust against short-term reductions in stress. This combination of opposite dynamic behaviors (reversible and irreversible) thus maximizes both adaptable and reliable decision-making over a broad range of changes in environmental conditions. These results suggest that decision-making systems might employ a general hybrid strategy to cope with unpredictably fluctuating environmental conditions

The Interplay between Surface Plasmon Resonance and Switching Properties in Gold@Spin Crossover Nanocomposites

Rod-like gold nanoparticles are directly embedded in a 1D-polymeric spin crossover (SCO) material leading to singular Au@SCO nanohybridarchitectures. The resulting architectures are designed to promote a synergetic effect between ultrafast spin-state photoswitching andphotothermal properties of plasmonic nanoparticles. This synergy is evidenced by the strong modulation of the surface plasmon resonance ofthe gold nanorods through the spin-state switching of the SCO component and also the strong enhancement of the photoswitching efficiency compared to pure SCO particles. This remarkable synergy results from the large modulation of the dielectric properties of the SCO polymer upon its thermal switching and the enhancement of the heating of these hybrid nanostructures upon excitation of the surface plasmon resonance of the gold nanorods

Clp ATPases are required for stress tolerance, intracellular replication and biofilm formation in Staphylococcus aureus

International audienceThe Hsp100/Clp ATPases constitute a family of closely related proteins of which some members function solely as chaperones whereas others additionally can associate with the unrelated ClpP peptidase forming a Clp proteolytic complex. We have investigated the role of four Clp ATPases in the versatile pathogen, Staphylococcus aureus. Previously, we showed that ClpX is required for expression of major virulence factors and for virulence of S. aureus, but not for survival during heat shock. In the present study, we have inactivated clpC, clpB and clpL and, while none of these mutations affected toxin production, both ClpC and ClpB and to a minor extent ClpL were required for intracellular multiplication within bovine mammary epithelial cells. These defects were paralleled by an inability of the clpC mutant to grow at high temperature and of the clpB mutant to induce thermotolerance indicating that the protective functions of these proteins are required both at high temperature and during infection. By primer extension analysis and footprint studies, we show that expression of clpC and clpB is controlled by the negative heat-shock regulator, CtsR, and that ClpC is required for its repressor activity. Thus, ClpC is a likely sensor of stress encountered during both environmental stress and infection. In addition to virulence factor production the ability to form biofilms is of importance to S. aureus as a nosocomial pathogen. Interestingly, biofilm formation was reduced in the absence of ClpX or ClpC whereas it was enhanced in the absence of ClpP. Thus, our data show that Clp proteolytic complexes and the Clp ATPases control several key processes of importance to the success of S. aureus as a pathogen