Growth rate regulation in Escherichia coli

Strength and Regulation of Seven rRNA Promoters in Escherichia coli

The model prokaryote Escherichia coli contains seven copies of the rRNA operon in the genome. The presence of multiple rRNA operons is an advantage for increasing the level of ribosome, the key apparatus of translation, in response to environmental conditions. The complete sequence of E. coli genome, however, indicated the micro heterogeneity between seven rRNA operons, raising the possibility in functional heterogeneity and/or differential mode of expression. The aim of this research is to determine the strength and regulation of the promoter of each rRNA operon in E. coli. For this purpose, we used the double-fluorescent protein reporter pBRP system that was developed for accurate and precise determination of the promoter strength of protein-coding genes. For application of this promoter assay vector for measurement of the rRNA operon promoters devoid of the signal for translation, a synthetic SD sequence was added at the initiation codon of the reporter GFP gene, and then approximately 500 bp-sequence upstream each 16S rRNA was inserted in front of this SD sequence. Using this modified pGRS system, the promoter activity of each rrn operon was determined by measuring the rrn promoter-directed GFP and the reference promoter-directed RFP fluorescence, both encoded by a single and the same vector. Results indicated that: the promoter activity was the highest for the rrnE promoter under all growth conditions analyzed, including different growth phases of wild-type E. coli grown in various media; but the promoter strength of other six rrn promoters was various depending on the culture conditions. These findings altogether indicate that seven rRNA operons are different with respect to the regulation mode of expression, conferring an advantage to E. coli through a more fine-tuned control of ribosome formation in a wide range of environmental situations. Possible difference in the functional role of each rRNA operon is also discussed.

First-Step Mutations during Adaptation Restore the Expression of Hundreds of Genes

The temporal change of phenotypes during the adaptive process remains largely unexplored, as do the genetic changes that affect these phenotypic changes. Here we focused on three mutations that rose to high frequency in the early stages of adaptation within 12 Escherichia coli populations subjected to thermal stress (42 °C). All the mutations were in the rpoB gene, which encodes the RNA polymerase beta subunit. For each mutation, we measured the growth curves and gene expression (mRNAseq) of clones at 42 °C. We also compared growth and gene expression with their ancestor under unstressed (37 °C) and stressed conditions (42 °C). Each of the three mutations changed the expression of hundreds of genes and conferred large fitness advantages, apparently through the restoration of global gene expression from the stressed toward the prestressed state. These three mutations had a similar effect on gene expression as another single mutation in a distinct domain of the rpoB protein. Finally, we compared the phenotypic characteristics of one mutant, I572L, with two high-temperature adapted clones that have this mutation plus additional background mutations. The background mutations increased fitness, but they did not substantially change gene expression. We conclude that early mutations in a global transcriptional regulator cause extensive changes in gene expression, many of which are likely under positive selection for their effect in restoring the prestress physiology.

Unveiling the Metabolic Pathways Associated with the Adaptive Reduction of Cell Size During Vibrio harveyi Persistence in Seawater Microcosms

Owing to their ubiquitous presence and ability to act as primary or opportunistic pathogens, Vibrio species greatly contribute to the diversity and evolution of marine ecosystems. This study was aimed at unveiling the cellular strategies enabling the marine gammaproteobacterium Vibrio harveyi to adapt and persist in natural aquatic systems. We found that, although V. harveyi incubation in seawater microcosm at 20 °C for 2 weeks did not change cell viability and culturability, it led to a progressive reduction in the average cell size. Microarray analysis revealed that this morphological change was accompanied by a profound decrease in gene expression affecting the central carbon metabolism, major biosynthetic pathways, and energy production. In contrast, V. harveyi elevated expression of genes closely linked to the composition and function of cell envelope. In addition to triggering lipid degradation via the β-oxidation pathway and apparently promoting the use of endogenous fatty acids as a major energy and carbon source, V. harveyi upregulated genes involved in ancillary mechanisms important for sustaining iron homeostasis, cell resistance to the toxic effect of reactive oxygen species, and recycling of amino acids. The above adaptation mechanisms and morphological changes appear to represent the major hallmarks of the initial V. harveyi response to starvation.

The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure?

The nucleolus is a prominent nuclear structure that is the site of ribosomal RNA (rRNA) transcription, and hence ribosome biogenesis. Cellular demand for ribosomes, and hence rRNA, is tightly linked to cell growth and the rRNA makes up the majority of all the RNA within a cell. To fulfill the cellular demand for rRNA, the ribosomal RNA (rDNA) genes are amplified to high copy number and transcribed at very high rates. As such, understanding the rDNA has profound consequences for our comprehension of genome and transcriptional organization in cells. In this review, we address the question of whether the nucleolus is a raft adrift the sea of nuclear DNA, or actively contributes to genome organization. We present evidence supporting the idea that the nucleolus, and the rDNA contained therein, play more roles in the biology of the cell than simply ribosome biogenesis. We propose that the nucleolus and the rDNA are central factors in the spatial organization of the genome, and that rapid alterations in nucleolar structure in response to changing conditions manifest themselves in altered genomic structures that have functional consequences. Finally, we discuss some predictions that result from the nucleolus having a central role in nuclear organization.

The dynamic nature and territory of transcriptional machinery in the bacterial chromosome

Our knowledge of the regulation of genes involved in bacterial growth and stress responses is extensive; however, we have only recently begun to understand how environmental cues influence the dynamic, three-dimensional distribution of RNA polymerase (RNAP) in Escherichia coli on the level of single cell, using wide-field fluorescence microscopy and state-of-the-art imaging techniques. Live-cell imaging using either an agarose-embedding procedure or a microfluidic system further underscores the dynamic nature of the distribution of RNAP in response to changes in the environment and highlights the challenges in the study. A general agreement between live-cell and fixed-cell images has validated the formaldehyde-fixing procedure, which is a technical breakthrough in the study of the cell biology of RNAP. In this review we use a systems biology perspective to summarize the advances in the cell biology of RNAP in E. coli, including the discoveries of the bacterial nucleolus, the spatial compartmentalization of the transcription machinery at the periphery of the nucleoid, and the segregation of the chromosome territories for the two major cellular functions of transcription and replication in fast-growing cells. Our understanding of the coupling of transcription and bacterial chromosome (or nucleoid) structure is also summarized. Using E. coli as a simple model system, co-imaging of RNAP with DNA and other factors during growth and stress responses will continue to be a useful tool for studying bacterial growth and adaptation in changing environment.

Spatial organization of transcription machinery and its segregation from the replisome in fast-growing bacterial cells

In a fast-growing Escherichia coli cell, most RNA polymerase (RNAP) is allocated to rRNA synthesis forming transcription foci at clusters of rrn operons or bacterial nucleolus, and each of the several nascent nucleoids contains multiple pairs of replication forks. The composition of transcription foci has not been determined. In addition, how the transcription machinery is three-dimensionally organized to promote cell growth in concord with replication machinery in the nucleoid remains essentially unknown. Here, we determine the spatial and functional landscapes of transcription and replication machineries in fast-growing E. coli cells using super-resolution-structured illumination microscopy. Co-images of RNAP and DNA reveal spatial compartmentation and duplication of the transcription foci at the surface of the bacterial chromosome, encompassing multiple nascent nucleoids. Transcription foci cluster with NusA and NusB, which are the rrn anti-termination system and are associated with nascent rRNAs. However, transcription foci tend to separate from SeqA and SSB foci, which track DNA replication forks and/or the replisomes, demonstrating that transcription machinery and replisome are mostly located in different chromosomal territories to maintain harmony between the two major cellular functions in fast-growing cells. Our study suggests that bacterial chromosomes are spatially and functionally organized, analogous to eukaryotes.

Perturbed states of the bacterial chromosome: a thymineless death case study

Spatial patterns of transcriptional activity in the living genome of Escherichia coli represent one of the more peculiar aspects of the E. coli chromosome biology. Spatial transcriptional correlations can be observed throughout the chromosome, and their formation depends on the state of replication in the cell. The condition of thymine starvation leading to thymineless death (TLD) is at the "cross-roads" of replication and transcription. According to a current view, e.g., (Cagliero et al., 2014), one of the cellular objectives is to segregate the processes of transcription and replication in time and space. An ultimate segregation would take place when one process is inhibited and another is not, as it happens during thymine starvation, which results in numerous molecular and physiological abnormalities associated with TLD. One of such abnormalities is the loss of spatial correlations in the vicinity of the origin of replication. We review the transcriptional consequences of replication inhibition by thymine starvation in a context of the state of DNA template in the starved cells and opine about a possible significance of normal physiological coupling between the processes of replication and transcription.

Engineered ribosomal RNA operon copy-number variants of E. coli reveal the evolutionary trade-offs shaping rRNA operon number

Ribosomal RNA (rrn) operons, characteristically present in several copies in bacterial genomes (7 in E. coli), play a central role in cellular physiology. We investigated the factors determining the optimal number of rrn operons in E. coli by constructing isogenic variants with 5–10 operons. We found that the total RNA and protein content, as well as the size of the cells reflected the number of rrn operons. While growth parameters showed only minor differences, competition experiments revealed a clear pattern: 7–8 copies were optimal under conditions of fluctuating, occasionally rich nutrient influx and lower numbers were favored in stable, nutrient-limited environments. We found that the advantages of quick adjustment to nutrient availability, rapid growth and economic regulation of ribosome number all contribute to the selection of the optimal rrn operon number. Our results suggest that the wt rrn operon number of E. coli reflects the natural, ‘feast and famine’ life-style of the bacterium, however, different copy numbers might be beneficial under different environmental conditions. Understanding the impact of the copy number of rrn operons on the fitness of the cell is an important step towards the creation of functional and robust genomes, the ultimate goal of synthetic biology.

In vivo kinetics of segregation and polar retention of MS2-GFP-RNA complexes in Escherichia coli

The cytoplasm of Escherichia coli is a crowded, heterogeneous environment. From single cell live imaging, we investigated the spatial kinetics and heterogeneities of synthetic RNA-protein complexes. First, although their known tendency to accumulate at the cell poles does not appear to introduce asymmetries between older and newer cell poles within a cell lifetime, these emerge with cell divisions. This suggests strong polar retention of the complexes, which we verified in their history of positions and mean escape time from the poles. Next, we show that the polar retention relies on anisotropies in the displacement distribution in the region between midcell and poles, whereas the speed is homogeneous along the major cell axis. Afterward, we establish that these regions are at the border of the nucleoid and shift outward with cell growth, due to the nucleoid's replication. Overall, the spatiotemporal kinetics of the complexes, which is robust to suboptimal temperatures, suggests that nucleoid occlusion is a source of dynamic heterogeneities of macromolecules in E. coli that ultimately generate phenotypic differences between sister cells.

The functional basis of adaptive evolution in chemostats

Two of the central problems in biology are determining the molecular basis of adaptive evolution and understanding how cells regulate their growth. The chemostat is a device for culturing cells that provides great utility in tackling both of these problems: it enables precise control of the selective pressure under which organisms evolve and it facilitates experimental control of cell growth rate. The aim of this review is to synthesize results from studies of the functional basis of adaptive evolution in long-term chemostat selections using Escherichia coli and Saccharomyces cerevisiae. We describe the principle of the chemostat, provide a summary of studies of experimental evolution in chemostats, and use these studies to assess our current understanding of selection in the chemostat. Functional studies of adaptive evolution in chemostats provide a unique means of interrogating the genetic networks that control cell growth, which complements functional genomic approaches and quantitative trait loci (QTL) mapping in natural populations. An integrated approach to the study of adaptive evolution that accounts for both molecular function and evolutionary processes is critical to advancing our understanding of evolution. By renewing efforts to integrate these two research programs, experimental evolution in chemostats is ideally suited to extending the functional synthesis to the study of genetic networks.

A genome-scale metabolic flux model of Escherichia coli K-12 derived from the EcoCyc database

BACKGROUND

Constraint-based models of Escherichia coli metabolic flux have played a key role in computational studies of cellular metabolism at the genome scale. We sought to develop a next-generation constraint-based E. coli model that achieved improved phenotypic prediction accuracy while being frequently updated and easy to use. We also sought to compare model predictions with experimental data to highlight open questions in E. coli biology.

RESULTS

We present EcoCyc-18.0-GEM, a genome-scale model of the E. coli K-12 MG1655 metabolic network. The model is automatically generated from the current state of EcoCyc using the MetaFlux software, enabling the release of multiple model updates per year. EcoCyc-18.0-GEM encompasses 1445 genes, 2286 unique metabolic reactions, and 1453 unique metabolites. We demonstrate a three-part validation of the model that breaks new ground in breadth and accuracy: (i) Comparison of simulated growth in aerobic and anaerobic glucose culture with experimental results from chemostat culture and simulation results from the E. coli modeling literature. (ii) Essentiality prediction for the 1445 genes represented in the model, in which EcoCyc-18.0-GEM achieves an improved accuracy of 95.2% in predicting the growth phenotype of experimental gene knockouts. (iii) Nutrient utilization predictions under 431 different media conditions, for which the model achieves an overall accuracy of 80.7%. The model's derivation from EcoCyc enables query and visualization via the EcoCyc website, facilitating model reuse and validation by inspection. We present an extensive investigation of disagreements between EcoCyc-18.0-GEM predictions and experimental data to highlight areas of interest to E. coli modelers and experimentalists, including 70 incorrect predictions of gene essentiality on glucose, 80 incorrect predictions of gene essentiality on glycerol, and 83 incorrect predictions of nutrient utilization.

CONCLUSION

Significant advantages can be derived from the combination of model organism databases and flux balance modeling represented by MetaFlux. Interpretation of the EcoCyc database as a flux balance model results in a highly accurate metabolic model and provides a rigorous consistency check for information stored in the database.

Source and regulation of flux variability in Escherichia coli

BACKGROUND

Metabolic responses are essential for the adaptation of microorganisms to changing environmental conditions. The repertoire of flux responses that the metabolic network can display in different external conditions may be quantified applying flux variability analysis to genome-scale metabolic reconstructions.

RESULTS

A procedure is developed to classify and quantify the sources of flux variability. We apply the procedure to the latest Escherichia coli metabolic reconstruction, in glucose minimal medium, with an additional constraint to account for the mechanism coordinating carbon and nitrogen utilization mediated by α-ketoglutarate. Flux variability can be decomposed into three components: internal, external and growth variability. Unexpectedly, growth variability is the only significant component of flux variability in the physiological ranges of glucose, oxygen and ammonia uptake rates. To obtain substantial increases in metabolic flexibility, E. coli must decrease growth rate to suboptimal values. This growth-flexibility trade-off gives a straightforward interpretation to recent work showing that most overall cell-to-cell flux variability in a population of E. coli can be attained sampling a small number of enzymes most likely to constrain cell growth. Importantly, it provides an explanation for the global reorganization occurring in metabolic networks during adaptations to environmental challenges. The calculations were repeated with a pathogenic strain and an old reconstruction of the commensal strain, having less than 50% of the reactions of the latest reconstruction, obtaining the same general conclusions.

CONCLUSIONS

In E. coli growing on glucose, growth variability is the only significant component of flux variability for all physiological conditions explored. Increasing flux variability requires reducing growth to suboptimal values. The growth-flexibility trade-off operates in physiological and evolutionary adaptations, and provides an explanation for the global reorganization occurring during adaptations to environmental challenges. The results obtained do not rely on the knowledge of kinetic and regulatory details of the system and are highly robust to incomplete or incorrect knowledge of the reaction network.

In vitro characterization of 6S RNA release-defective mutants uncovers features of pRNA-dependent release from RNA polymerase in E. coli

6S RNA is a noncoding RNA that inhibits bacterial transcription by sequestering RNA polymerase holoenzyme (Eσ(70)) in low-nutrient conditions. This transcriptional block can be relieved by the synthesis of a short product RNA (pRNA) using the 6S RNA as a template. Here, we selected a range of 6S RNA release-defective mutants from a high diversity in vitro pool. Studying the release-defective variant R9-33 uncovered complex interactions between three regions of the 6S RNA. As expected, mutating the transcriptional start site (TSS) slowed and partially inhibited release. Surprisingly, additional mutations near the TSS were found that rescued this effect. Likewise, three mutations in the top strand of the large open bubble (LOB) could considerably slow release but were rescued by the addition of upstream mutations found between a highly conserved "-35" motif and the LOB. Combining the three top strand LOB mutations with mutations near the TSS, however, was particularly effective at preventing release, and this effect could be further enhanced by inclusion of the upstream mutations. Overexpressing R9-33 and a series of milder release-defective mutants in Escherichia coli resulted in a delayed entry into exponential phase together with a decrease in cell survival that correlated well with the severity of the in vitro phenotypes. The complex crosstalk observed between distinct regions of the 6S RNA supports a scrunching type model of 6S RNA release, where at least three regions of the 6S RNA must interact with Eσ(70) in a cooperative manner so as to ensure effective pRNA-dependent release.

RNase III: Genetics and function; structure and mechanism

RNase III is a global regulator of gene expression in Escherichia coli that is instrumental in the maturation of ribosomal and other structural RNAs. We examine here how RNase III itself is regulated in response to growth and other environmental changes encountered by the cell and how, by binding or processing double-stranded RNA (dsRNA) intermediates, RNase III controls the expression of genes. Recent insight into the mechanism of dsRNA binding and processing, gained from structural studies of RNase III, is reviewed. Structural studies also reveal new cleavage sites in the enzyme that can generate longer 3' overhangs.

AspC-mediated aspartate metabolism coordinates the Escherichia coli cell cycle

Background

The fast-growing bacterial cell cycle consists of at least two independent cycles of chromosome replication and cell division. To ensure proper cell cycles and viability, chromosome replication and cell division must be coordinated. It has been suggested that metabolism could affect the Escherichia coli cell cycle, but the idea is still lacking solid evidences.

Methodology/Principle Findings

We found that absence of AspC, an aminotransferase that catalyzes synthesis of aspartate, led to generation of small cells with less origins and slow growth. In contrast, excess AspC was found to exert the opposite effect. Further analysis showed that AspC-mediated aspartate metabolism had a specific effect in the cell cycle, as only extra aspartate of the 20 amino acids triggered production of bigger cells with more origins per cell and faster growth. The amount of DnaA protein per cell was found to be changed in response to the availability of AspC. Depletion of (p)ppGpp by ΔrelAΔspoT led to a slight delay in initiation of replication, but did not change the replication pattern found in the ΔaspC mutant.

Conclusion/Significances

The results suggest that AspC-mediated metabolism of aspartate coordinates the E. coli cell cycle through altering the amount of the initiator protein DnaA per cell and the division signal UDP-glucose. Furthermore, AspC sequence conservation suggests similar functions in other organisms.

Escherichia coli achieves faster growth by increasing catalytic and translation rates of proteins

Regulation levels of the gene expression cascade controlling protein levels and metabolic fluxes for cells to achieve faster growth have not been elaborated in acceptable detail. Furthermore, there is need for specific growth rate (μ) dependent absolute quantitative transcriptome and proteome data to understand the molecular relationships for enabling cells to modify μ. We address these questions, for the first time, by presenting regulatory strategies for more efficient metabolism of Escherichia coli at higher μ by statistical covariance analysis of genome-wide intracellular mRNA and protein concentrations coupled to metabolic flux analysis in the steady state range of μ = 0.11-0.49 h(-1). Our analyses show dominating post-transcriptional control of protein abundances and post-translational control of flux rates. On average, E. coli achieved five-times faster growth through 3.7-fold increase of apparent catalytic rates of enzymes (kapp) and 2.5-fold increased translation rates, demonstrating the relevance of post-translational regulation for increasing flux throughput. Interestingly, pathways carrying the highest flux showed both high protein abundance and kapp values. Furthermore, co-regulation analysis of enzymatic capacities revealed tightly coupled regulatory dependencies of protein synthesis and RNA precursor synthesis, substrate utilization, biosynthetic and energy generation pathways carrying the highest flux. We also observed metabolic pathway and COG specific protein and metabolic flux control levels, protein expression costs and genome-wide principles for translation efficiency and transcription unit polarity. This work contributes to the much needed quantitative understanding of coordinated gene expression regulation and metabolic flux control. Our findings will also advance modeling and metabolic engineering of industrial strains.

Heteroresistance at the single-cell level: adapting to antibiotic stress through a population-based strategy and growth-controlled interphenotypic coordination

Heteroresistance refers to phenotypic heterogeneity of microbial clonal populations under antibiotic stress, and it has been thought to be an allocation of a subset of “resistant” cells for surviving in higher concentrations of antibiotic. The assumption fits the so-called bet-hedging strategy, where a bacterial population “hedges” its “bet” on different phenotypes to be selected by unpredicted environment stresses. To test this hypothesis, we constructed a heteroresistance model by introducing a bla_CTX-M-14 gene (coding for a cephalosporin hydrolase) into a sensitive Escherichia coli strain. We confirmed heteroresistance in this clone and that a subset of the cells expressed more hydrolase and formed more colonies in the presence of ceftriaxone (exhibited stronger “resistance”). However, subsequent single-cell-level investigation by using a microfluidic device showed that a subset of cells with a distinguishable phenotype of slowed growth and intensified hydrolase expression emerged, and they were not positively selected but increased their proportion in the population with ascending antibiotic concentrations. Therefore, heteroresistance—the gradually decreased colony-forming capability in the presence of antibiotic—was a result of a decreased growth rate rather than of selection for resistant cells. Using a mock strain without the resistance gene, we further demonstrated the existence of two nested growth-centric feedback loops that control the expression of the hydrolase and maximize population growth in various antibiotic concentrations. In conclusion, phenotypic heterogeneity is a population-based strategy beneficial for bacterial survival and propagation through task allocation and interphenotypic collaboration, and the growth rate provides a critical control for the expression of stress-related genes and an essential mechanism in responding to environmental stresses.

Heteroresistance is essentially phenotypic heterogeneity, where a population-based strategy is thought to be at work, being assumed to be variable cell-to-cell resistance to be selected under antibiotic stress. Exact mechanisms of heteroresistance and its roles in adaptation to antibiotic stress have yet to be fully understood at the molecular and single-cell levels. In our study, we have not been able to detect any apparent subset of “resistant” cells selected by antibiotics; on the contrary, cell populations differentiate into phenotypic subsets with variable growth statuses and hydrolase expression. The growth rate appears to be sensitive to stress intensity and plays a key role in controlling hydrolase expression at both the bulk population and single-cell levels. We have shown here, for the first time, that phenotypic heterogeneity can be beneficial to a growing bacterial population through task allocation and interphenotypic collaboration other than partitioning cells into different categories of selective advantage.

Dual role of transcription and transcript stability in the regulation of gene expression in Escherichia coli cells cultured on glucose at different growth rates

Microorganisms extensively reorganize gene expression to adjust growth rate to changes in growth conditions. At the genomic scale, we measured the contribution of both transcription and transcript stability to regulating messenger RNA (mRNA) concentration in Escherichia coli. Transcriptional control was the dominant regulatory process. Between growth rates of 0.10 and 0.63 h(-1), there was a generic increase in the bulk mRNA transcription. However, many transcripts became less stable and the median mRNA half-life decreased from 4.2 to 2.8 min. This is the first evidence that mRNA turnover is slower at extremely low-growth rates. The destabilization of many, but not all, transcripts at high-growth rate correlated with transcriptional upregulation of genes encoding the mRNA degradation machinery. We identified five classes of growth-rate regulation ranging from mainly transcriptional to mainly degradational. In general, differential stability within polycistronic messages encoded by operons does not appear to be affected by growth rate. We show here that the substantial reorganization of gene expression involving downregulation of tricarboxylic acid cycle genes and acetyl-CoA synthetase at high-growth rates is controlled mainly by transcript stability. Overall, our results demonstrate that the control of transcript stability has an important role in fine-tuning mRNA concentration during changes in growth rate.

Bacterial virulence and Fis: adapting regulatory networks to the host environment

Pathogenic bacteria have to cope with adverse conditions, such as the host environment and host defense reactions. To adapt quickly to environmental changes, pathogens have developed complex regulatory networks that ensure adequate expression of their virulence genes. Recent evidence suggests that Fis, an abundant nucleoid-associated protein transiently produced during early exponential growth, plays a major role in these networks in several pathogenic bacteria. This review focuses on two enterobacteria, Salmonella enterica and Dickeya dadantii, that inhabit distinct ecological niches to illustrate how Fis uses different strategies to coordinate virulence gene expression, depending on the bacterial lifestyle.

Transmission of an oxygen availability signal at the Salmonella enterica serovar Typhimurium fis promoter

The nucleoid-associated protein FIS is a global regulator of gene expression and chromosome structure in Escherichia coli and Salmonella enterica. Despite the importance of FIS for infection and intracellular invasion, very little is known about the regulation of S. enterica fis expression. Under standard laboratory growth conditions, fis is highly expressed during rapid growth but is then silenced as growth slows. However, if cells are cultured in non-aerated conditions, fis expression is sustained during stationary phase. This led us to test whether the redox-sensing transcription factors ArcA and FNR regulate S. enterica fis. Deletion of FNR had no detectable effect, whereas deletion of ArcA had the unexpected effect of further elevating fis expression in stationary phase. ArcA required RpoS for induction of fis expression, suggesting that ArcA indirectly affects fis expression. Other putative regulators were found to play diverse roles: FIS acted directly as an auto-repressor (as expected), whereas CRP had little direct effect on fis expression. Deleting regions of the fis promoter led to the discovery of a novel anaerobically-induced transcription start site (Pfis-2) upstream of the primary transcription start site (Pfis-1). Promoter truncation also revealed that the shortest functional fis promoter was incapable of sustained expression. Moreover, fis expression was observed to correlate directly with DNA supercoiling in non-aerated conditions. Thus, the full-length S. enterica fis promoter region may act as a topological switch that is sensitive to stress-induced duplex destabilisation and up-regulates expression in non-aerated conditions.

Direct observation of single stationary-phase bacteria reveals a surprisingly long period of constant protein production activity

Exponentially growing bacteria are rarely found in the wild, as microorganisms tend to spend most of their lifetime at stationary phase. Despite this general prevalence of stationary-phase bacteria, they are as yet poorly characterized. Our goal was to quantitatively study this phase by direct observation of single bacteria as they enter into stationary phase and by monitoring their activity over several days during growth arrest. For this purpose, we devised an experimental procedure for starving single Escherichia coli bacteria in microfluidic devices and measured their activity by monitoring the production rate of fluorescent proteins. When amino acids were the sole carbon source, the production rate decreased by an order of magnitude upon entry into stationary phase. We found that, even while growth-arrested, bacteria continued to produce proteins at a surprisingly constant rate over several days. Our identification of this newly observed period of constant activity in nongrowing cells, designated as constant activity stationary phase, makes possible the conduction of assays that require constant protein expression over time, and are therefore difficult to perform under exponential growth conditions. Moreover, we show that exogenous protein expression bears no fitness cost on the regrowth of the population when starvation ends. Further characterization of constant activity stationary phase-a phase where nongrowing bacteria can be quantitatively studied over several days in a reproducible manner-should contribute to a better understanding of this ubiquitous but overlooked physiological state of bacteria in nature.

Carbon catabolite repression correlates with the maintenance of near invariant molecular crowding in proliferating E. coli cells

BACKGROUND

Carbon catabolite repression (CCR) is critical for optimal bacterial growth, and in bacterial (and yeast) cells it leads to their selective consumption of a single substrate from a complex environment. However, the root cause(s) for the development of this regulatory mechanism is unknown. Previously, a flux balance model (FBAwMC) of Escherichia coli metabolism that takes into account the crowded intracellular milieu of the bacterial cell correctly predicted selective glucose uptake in a medium containing five different carbon sources, suggesting that CCR may be an adaptive mechanism that ensures optimal bacterial metabolic network activity for growth.

RESULTS

Here, we show that slowly growing E. coli cells do not display CCR in a mixed substrate culture and gradual activation of CCR correlates with an increasing rate of E. coli cell growth and proliferation. In contrast, CCR mutant cells do not achieve fast growth in mixed substrate culture, and display differences in their cell volume and density compared to wild-type cells. Analyses of transcriptome data from wt E. coli cells indicate the expected regulation of substrate uptake and metabolic pathway utilization upon growth rate change. We also find that forced transient increase of intracellular crowding or transient perturbation of CCR delay cell growth, the latter leading to associated cell density-and volume alterations.

CONCLUSIONS

CCR is activated at an increased bacterial cell growth rate when it is required for optimal cell growth while intracellular macromolecular density is maintained within a narrow physiological range. In addition to CCR, there are likely to be other regulatory mechanisms of cell metabolism that have evolved to ensure optimal cell growth in the context of the fundamental biophysical constraint imposed by intracellular molecular crowding.

Growth rate-coordinated transcriptome reorganization in bacteria

Background

Cell growth rate reflects an organism’s physiological state and largely relies on the ability of gene expression to respond to the environment. The relationship between cellular growth rate and gene expression remains unknown.

Results

Growth rate-coordinated changes in gene expression were discovered by analyzing exponentially growing Escherichia coli cells cultured under multiple defined environments, in which osmotic pressure, temperature and starvation status were varied. Gene expression analyses showed that all 3,740 genes in the genome could be simply divided into three clusters (C1, C2 and C3), which were accompanied by a generic trend in the growth rate that was coordinated with transcriptional changes. The direction of transcriptional change in C1 indicated environmental specificity, whereas those in C2 and C3 were correlated negatively and positively with growth rates, respectively. The three clusters exhibited differentiated gene functions and gene regulation task division.

Conclusions

We identified three gene clusters, exhibiting differential gene functions and distinct directions in their correlations with growth rates. Reverses in the direction of the growth rate correlated transcriptional changes and the distinguished duties of the three clusters indicated how transcriptome homeostasis is maintained to balance the total expression cost for sustaining life in new habitats.

The significance of translation regulation in the stress response

Background

The stress response in bacteria involves the multistage control of gene expression but is not entirely understood. To identify the translational response of bacteria in stress conditions and assess its contribution to the regulation of gene expression, the translational states of all mRNAs were compared under optimal growth condition and during nutrient (isoleucine) starvation.

Results

A genome-scale study of the translational response to nutritional limitation was performed in the model bacterium Lactococcus lactis. Two measures were used to assess the translational status of each individual mRNA: the fraction engaged in translation (ribosome occupancy) and ribosome density (number of ribosomes per 100 nucleotides). Under isoleucine starvation, half of the mRNAs considered were translationally down-regulated mainly due to decreased ribosome density. This pattern concerned genes involved in growth-related functions such as translation, transcription, and the metabolism of fatty acids, phospholipids and bases, contributing to the slowdown of growth. Only 4% of the mRNAs were translationally up-regulated, mostly related to prophagic expression in response to stress. The remaining genes exhibited antagonistic regulations of the two markers of translation. Ribosome occupancy increased significantly for all the genes involved in the biosynthesis of isoleucine, although their ribosome density had decreased. The results revealed complex translational regulation of this pathway, essential to cope with isoleucine starvation.

To elucidate the regulation of global gene expression more generally, translational regulation was compared to transcriptional regulation under isoleucine starvation and to other post-transcriptional regulations related to mRNA degradation and mRNA dilution by growth. Translational regulation appeared to accentuate the effects of transcriptional changes for down-regulated growth-related functions under isoleucine starvation although mRNA stabilization and lower dilution by growth counterbalanced this effect.

Conclusions

We show that the contribution of translational regulation to the control of gene expression is significant in the stress response. Post-transcriptional regulation is complex and not systematically co-directional with transcription regulation. Post-transcriptional regulation is important to the understanding of gene expression control.

Epigenetic feedback regulation accelerates adaptation and evolution

A simple cell model consisting of a gene regulatory network with epigenetic feedback regulation is studied to evaluate the effect of epigenetic dynamics on adaptation and evolution. We find that, the type of epigenetic dynamics considered enables a cell to adapt to unfamiliar environmental changes, for which no regulatory program has been prepared, through noise-driven selection of a cellular state with a high growth rate. Furthermore, we demonstrate that the inclusion of epigenetic regulation promotes evolutionary development of a regulatory network that can respond to environmental changes in a fast and precise manner. These results strongly suggest that epigenetic feedback regulation in gene expression dynamics provides a significant increase in fitness by engendering an increase in cellular plasticity during adaptation and evolution.

Genome conformation capture reveals that the Escherichia coli chromosome is organized by replication and transcription

To fit within the confines of the cell, bacterial chromosomes are highly condensed into a structure called the nucleoid. Despite the high degree of compaction in the nucleoid, the genome remains accessible to essential biological processes, such as replication and transcription. Here, we present the first high-resolution chromosome conformation capture-based molecular analysis of the spatial organization of the Escherichia coli nucleoid during rapid growth in rich medium and following an induced amino acid starvation that promotes the stringent response. Our analyses identify the presence of origin and terminus domains in exponentially growing cells. Moreover, we observe an increased number of interactions within the origin domain and significant clustering of SeqA-binding sequences, suggesting a role for SeqA in clustering of newly replicated chromosomes. By contrast, ‘histone-like’ protein (i.e. Fis, IHF and H-NS) -binding sites did not cluster, and their role in global nucleoid organization does not manifest through the mediation of chromosomal contacts. Finally, genes that were downregulated after induction of the stringent response were spatially clustered, indicating that transcription in E. coli occurs at transcription foci.

Dissociation and re-association of RNA polymerase with DNA during osmotic stress response in Escherichia coli

The thermodynamic association of RNA polymerase (RNAP) with DNA is sensitive to salt concentration in vitro. Paradoxically, previous studies of changes in osmolarity during steady-state cell growth found no dependence between the association of RNAP to DNA and K(+) concentration in Escherichia coli. We reevaluated this issue by following the interaction of RNAP and genomic DNA in time-course experiments during the hyper-osmotic response. Our results show that the interaction is temporally controlled by the same physical chemistry principle in the cell as in vitro. RNAP rapidly dissociates from the genome during the initial response when the cytoplasmic K(+) accumulates transiently, and concurrently the nucleoid becomes hyper-condensed. The freed RNAP re-associates with the genome during a subsequent osmoadaptation phase when organic osmoprotectants accumulate as K(+) levels decrease. RNAP first surrounds the hyper-condensed nucleoid forming a sphere of RNAP before it progressively moves in to the center of the nucleoid. Our findings reinterpret the dynamic protein-DNA interactions during osmotic stress response. We discuss the implications of the dissociation/association of RNAP for osmotic protection and nucleoid structure.

A single-cell perspective on non-growing but metabolically active (NGMA) bacteria

A long-standing and fundamental problem in microbiology is the non-trivial discrimination between live and dead cells. The existence of physically intact and possibly viable bacterial cells that fail to replicate during a more or less protracted period of observation, despite environmental conditions that are ostensibly propitious for growth, has been extensively documented in many different organisms. In clinical settings, non-culturable cells may contribute to non-apparent infections capable of reactivating after months or years of clinical latency, a phenomenon that has been well documented in the specific case of Mycobacterium tuberculosis. The prevalence of these silent but potentially problematic bacterial reservoirs has been highlighted by classical approaches such as limiting culture dilution till extinction of growing cells, followed by resuscitation of apparently "viable but non-culturable" (VBNC) subpopulations. Although these assays are useful to demonstrate the presence of VBNC cells in a population, they are effectively retrospective and are not well suited to the analysis of non-replicating cells per se. Here, we argue that research on a closely related problem, which we shall refer to as the "non-growing but metabolically active" state, is poised to advance rapidly thanks to the recent development of novel technologies and methods for real-time single-cell analysis. In particular, the combination of fluorescent reporter dyes and strains, microfluidic and microelectromechanical systems, and time-lapse fluorescence microscopy offers tremendous and largely untapped potential for future exploration of the physiology of non-replicating cells.

Chromosome Replication in Escherichia coli: Life on the Scales

At all levels of Life, systems evolve on the 'scales of equilibria'. At the level of bacteria, the individual cell must favor one of two opposing strategies and either take risks to grow or avoid risks to survive. It has been proposed in the Dualism hypothesis that the growth and survival strategies depend on non-equilibrium and equilibrium hyperstructures, respectively. It has been further proposed that the cell cycle itself is the way cells manage to balance the ratios of these types of hyperstructure so as to achieve the compromise solution of living on the two scales. Here, we attempt to re-interpret a major event, the initiation of chromosome replication in Escherichia coli, in the light of scales of equilibria. This entails thinking in terms of hyperstructures as responsible for intensity sensing and quantity sensing and how this sensing might help explain the role of the DnaA protein in initiation of replication. We outline experiments and an automaton approach to the cell cycle that should test and refine the scales concept.

T7 RNA polymerase functions in vitro without clustering

Many nucleic acid polymerases function in clusters known as factories. We investigate whether the RNA polymerase (RNAP) of phage T7 also clusters when active. Using ‘pulldowns’ and fluorescence correlation spectroscopy we find that elongation complexes do not interact in vitro with a K_d<1 µM. Chromosome conformation capture also reveals that genes located 100 kb apart on the E. coli chromosome do not associate more frequently when transcribed by T7 RNAP. We conclude that if clustering does occur in vivo, it must be driven by weak interactions, or mediated by a phage-encoded protein.

Regulation of expression and catalytic activity of Escherichia coli RsmG methyltransferase

RsmG is an AdoMet-dependent methyltransferase responsible for the synthesis of m(7)G527 in the 530 loop of bacterial 16S rRNA. This loop is universally conserved, plays a key role in ribosomal accuracy, and is a target for streptomycin binding. Loss of the m(7)G527 modification confers low-level streptomycin resistance and may affect ribosomal functioning. Here, we explore the mechanisms controlling RsmG expression and activity, which may somehow respond to the demand set by the amount of rRNA. We confirm that rsmG is the second member in a bicistronic operon and demonstrate that rsmG also has its own promoter, which appears, in actively growing cells, as a control device to offset both the relatively low stability of RsmG and inhibition of the operon promoter. RsmG levels decrease under conditions that down-regulate rRNA synthesis. However, coordination between rRNA and RsmG expression does not seem to occur at the level of transcription initiation. Instead, it might depend on the activity of an inverted repeated region, located between the rsmG promoter and ribosome binding site, which we show to work as a weak transcriptional terminator. To gain insights into the enzymatic mechanism of RsmG, highly conserved residues were mutated and the abilities of the resulting proteins to confer streptomycin resistance, to modify rRNA, and to bind AdoMet were explored. Our data demonstrate for the first time the critical importance of some residues located in the active site of Escherichia coli RsmG for the m(7)G modification process and suggest a role for them in rRNA binding and catalysis.