Dependence of transcription-coupled DNA supercoiling on promoter strength in Escherichia coli topoisomerase I deficient strains

Manipulation of topoisomerase expression inhibits cell division but not growth and reveals a distinctive promoter structure in Synechocystis

In cyanobacteria DNA supercoiling varies over the diurnal cycle and is integrated with temporal programs of transcription and replication. We manipulated DNA supercoiling in Synechocystis sp. PCC 6803 by CRISPRi-based knockdown of gyrase subunits and overexpression of topoisomerase I (TopoI). Cell division was blocked but cell growth continued in all strains. The small endogenous plasmids were only transiently relaxed, then became strongly supercoiled in the TopoI overexpression strain. Transcript abundances showed a pronounced 5'/3' gradient along transcription units, incl. the rRNA genes, in the gyrase knockdown strains. These observations are consistent with the basic tenets of the homeostasis and twin-domain models of supercoiling in bacteria. TopoI induction initially led to downregulation of G+C-rich and upregulation of A+T-rich genes. The transcriptional response quickly bifurcated into six groups which overlap with diurnally co-expressed gene groups. Each group shows distinct deviations from a common core promoter structure, where helically phased A-tracts are in phase with the transcription start site. Together, our data show that major co-expression groups (regulons) in Synechocystis all respond differentially to DNA supercoiling, and suggest to re-evaluate the long-standing question of the role of A-tracts in bacterial promoters.

CRISPR-Cas, DNA Supercoiling, and Nucleoid-Associated Proteins

In this opinion article we highlight links between the H-NS nucleoid-associated protein, variable DNA topology, the regulation of CRISPR-cas locus expression, CRISPR-Cas activity, and the recruitment of novel genetic information by the CRISPR array. We propose that the requirement that the invading mobile genetic element be negatively supercoiled limits effective CRISPR action to a window in the bacterial growth cycle when DNA topology is optimal, and that this same window is used for the efficient integration of new spacer sequences at the CRISPR array. H-NS silences CRISPR promoters, and we propose that antagonists of H-NS, such as the LeuO transcription factor, provide a basis for a stochastic genetic switch that acts at random in each cell in the bacterial population. In addition, we wish to propose a mechanism by which mobile genetic elements can suppress CRISPR-cas transcription using H-NS homologues. Although the individual components of this network are known, we propose a new model in which they are integrated and linked to the physiological state of the bacterium. The model provides a basis for cell-to-cell variation in the expression and performance of CRISPR systems in bacterial populations.

DNA supercoiling and transcription in bacteria: a two-way street

BACKGROUND

The processes of DNA supercoiling and transcription are interdependent because the movement of a transcription elongation complex simultaneously induces under- and overwinding of the DNA duplex and because the initiation, elongation and termination steps of transcription are all sensitive to the topological state of the DNA.

RESULTS

Policing of the local and global supercoiling of DNA by topoisomerases helps to sustain the major DNA-based transactions by eliminating barriers to the movement of transcription complexes and replisomes. Recent data from whole-genome and single-molecule studies have provided new insights into how interactions between transcription and the supercoiling of DNA influence the architecture of the chromosome and how they create cell-to-cell diversity at the level of gene expression through transcription bursting.

CONCLUSIONS

These insights into fundamental molecular processes reveal mechanisms by which bacteria can prevail in unpredictable and often hostile environments by becoming unpredictable themselves.

Inhibition of the gyrA promoter by transcription-coupled DNA supercoiling in Escherichia coli

The E. coli gyrA promoter (P_gyrA) is a DNA supercoiling sensitive promoter, stimulated by relaxation of DNA templates, and inhibited by (−) DNA supercoiling in bacteria. However, whether P_gyrA can be inhibited by transient and localized transcription-coupled DNA supercoiling (TCDS) has not been fully examined. In this paper, using different DNA templates including the E. coli chromosome, we show that transient and localized TCDS strongly inhibits P_gyrA in E. coli. This result can be explained by a twin-supercoiled domain model of transcription in which (+) and (−) supercoiled domains are generated around the transcribing RNA polymerase. We also find that fluoroquinolones, such as ciprofloxacin, can substantially increase the expression of the firefly luciferase under the control of the P_gyrA coupled to a divergent IPTG-inducible promoter in the presence of IPTG. This stimulation of P_gyrA by fluoroquinolones can be also explained by the twin-supercoiled domain model of transcription. This unique property of TCDS may be configured into a high throughput-screening (HTS) assay to identify antimicrobial compounds targeting bacterial DNA gyrase.

Model-based genome-wide determination of RNA chain elongation rates in Escherichia coli

Dynamics in the process of transcription are often simplified, yet they play an important role in transcript folding, translation into functional protein and DNA supercoiling. While the modulation of the speed of transcription of individual genes and its role in regulation and proper protein folding has been analyzed in depth, the functional relevance of differences in transcription speeds as well as the factors influencing it have not yet been determined on a genome-wide scale. Here we determined transcription speeds for the majority of E. coli genes based on experimental data. We find large differences in transcription speed between individual genes and a strong influence of both cellular location as well as the relative importance of genes for cellular function on transcription speeds. Investigating factors influencing transcription speeds we observe both codon composition as well as factors associated to DNA topology as most important factors influencing transcription speeds. Moreover, we show that differences in transcription speeds are sufficient to explain the timing of regulatory responses during environmental shifts and highlight the importance of the consideration of transcription speeds in the design of experiments measuring transcriptomic responses to perturbations.

Chromosomal organization of transcription: in a nutshell

Early studies of transcriptional regulation focused on individual gene promoters defined specific transcription factors as central agents of genetic control. However, recent genome-wide data propelled a different view by linking spatially organized gene expression patterns to chromosomal dynamics. Therefore, the major problem in contemporary molecular genetics concerned with transcriptional gene regulation is to establish a unifying model that reconciles these two views. This problem, situated at the interface of polymer physics and network theory, requires development of an integrative methodology. In this review, we discuss recent achievements in classical model organism E. coli and provide some novel insights gained from studies of a bacterial plant pathogen, D. dadantii. We consider DNA topology and the basal transcription machinery as key actors of regulation, in which activation of functionally relevant genes is coupled to and coordinated with the establishment of extended chromosomal domains of coherent transcription. We argue that the spatial organization of genome plays a fundamental role in its own regulation.

Alternative transcriptional regulation in genome-reduced bacteria

Transcription is a core process of bacterial physiology, and as such it must be tightly controlled, so that bacterial cells maintain steady levels of each RNA molecule in homeostasis and modify them in response to perturbations. The major regulators of transcription in bacteria (and in eukaryotes) are transcription factors. However, in genome-reduced bacteria, the limited number of these proteins is insufficient to explain the variety of responses shown upon changes in their environment. Thus, alternative regulators may play a central role in orchestrating RNA levels in these microorganisms. These alternative mechanisms rely on intrinsic features within DNA and RNA molecules, suggesting they are ancestral mechanisms shared among bacteria that could have an increased relevance on transcriptional regulation in minimal cells. In this review, we summarize the alternative elements that can regulate transcript abundance in genome-reduced bacteria and how they contribute to the RNA homeostasis at different levels.

Transient and dynamic DNA supercoiling potently stimulates the leu-500 promoter in Escherichia coli

The inactive prokaryotic leu-500 promoter (P_leu-500) contains a single A-to-G point mutation in the -10 region of the leucine operon promoter, which causes leucine auxotrophy. This promoter can be activated by (-) DNA supercoiling in Escherichia coli topA strains. However, whether this activation arises from global, permanent, or transient, dynamic supercoiling is still not fully understood. In this article, using a newly established in vivo system carrying a pair of divergently coupled promoters, i.e. an IPTG-inducible promoter and P_leu-500 that control the expression of lacZ and luc (the firefly luciferase gene), respectively, we demonstrate that transient, dynamic (-) DNA supercoiling provided by divergent transcription in both wild-type and topA strains can potently activate P_leu-500 We found that this activation depended on the promoter strength and the length of RNA transcripts, which are functional characteristics of transcription-coupled DNA supercoiling (TCDS) precisely predicted by the twin-supercoiled domain model of transcription in which a (+) supercoiled domain is produced ahead of the RNA polymerase and a (-) supercoiled domain behind it. We also demonstrate that TCDS can be generated on topologically open DNA molecules, i.e. linear DNA molecules, in Escherichia coli, suggesting that topological boundaries or barriers are not required for the production of TCDS in vivo This work demonstrates that transient, dynamic TCDS by RNA polymerases is a major chromosome remodeling force in E. coli and greatly influences the nearby, coupled promoters/transcription.

Large-Scale Conformational Transitions in Supercoiled DNA Revealed by Coarse-Grained Simulation

Topological constraints, such as those associated with DNA supercoiling, play an integral role in genomic regulation and organization in living systems. However, physical understanding of the principles that underlie DNA organization at biologically relevant length scales remains a formidable challenge. We develop a coarse-grained simulation approach for predicting equilibrium conformations of supercoiled DNA. Our methodology enables the study of supercoiled DNA molecules at greater length scales and supercoiling densities than previously explored by simulation. With this approach, we study the conformational transitions that arise due to supercoiling across the full range of supercoiling densities that are commonly explored by living systems. Simulations of ring DNA molecules with lengths at the scale of topological domains in the Escherichia coli chromosome (∼10 kilobases) reveal large-scale conformational transitions elicited by supercoiling. The conformational transitions result in three supercoiling conformational regimes that are governed by a competition among chiral coils, extended plectonemes, and branched hyper-supercoils. These results capture the nonmonotonic relationship of size versus degree of supercoiling observed in experimental sedimentation studies of supercoiled DNA, and our results provide a physical explanation of the conformational transitions underlying this behavior. The length scales and supercoiling regimes investigated here coincide with those relevant to transcription-coupled remodeling of supercoiled topological domains, and we discuss possible implications of these findings in terms of the interplay between transcription and topology in bacterial chromosome organization.

Inter-sigmulon communication through topological promoter coupling

Divergent transcription from within bacterial intergenic regions frequently involves promoters dependent on alternative σ-factors. This is the case for the non-overlapping σ70- and σ54-dependent promoters that control production of the substrate-responsive regulator and enzymes for (methyl)phenol catabolism. Here, using an array of in vivo and in vitro assays, we identify transcription-driven supercoiling arising from the σ54-promoter as the mechanism underlying inter-promoter communication that results in stimulation of the activity of the σ70-promoter. The non-overlapping 'back-to-back' configuration of a powerful σ54-promoter and weak σ70-promoter within this system offers a previously unknown means of inter-sigmulon communication that renders the σ70-promoter subservient to signals that elicit σ54-dependent transcription without it possessing a cognate binding site for the σ54-RNA polymerase holoenzyme. This mode of control has the potential to be a prevalent, but hitherto unappreciated, mechanism by which bacteria adjust promoter activity to gain appropriate transcriptional control.

DNA supercoiling is a fundamental regulatory principle in the control of bacterial gene expression

Although it has become routine to consider DNA in terms of its role as a carrier of genetic information, it is also an important contributor to the control of gene expression. This regulatory principle arises from its structural properties. DNA is maintained in an underwound state in most bacterial cells and this has important implications both for DNA storage in the nucleoid and for the expression of genetic information. Underwinding of the DNA through reduction in its linking number potentially imparts energy to the duplex that is available to drive DNA transactions, such as transcription, replication and recombination. The topological state of DNA also influences its affinity for some DNA binding proteins, especially in DNA sequences that have a high A + T base content. The underwinding of DNA by the ATP-dependent topoisomerase DNA gyrase creates a continuum between metabolic flux, DNA topology and gene expression that underpins the global response of the genome to changes in the intracellular and external environments. These connections describe a fundamental and generalised mechanism affecting global gene expression that underlies the specific control of transcription operating through conventional transcription factors. This mechanism also provides a basal level of control for genes acquired by horizontal DNA transfer, assisting microbial evolution, including the evolution of pathogenic bacteria.

DNA supercoiling is a fundamental regulatory principle in the control of bacterial gene expression

Although it has become routine to consider DNA in terms of its role as a carrier of genetic information, it is also an important contributor to the control of gene expression. This regulatory principle arises from its structural properties. DNA is maintained in an underwound state in most bacterial cells and this has important implications both for DNA storage in the nucleoid and for the expression of genetic information. Underwinding of the DNA through reduction in its linking number potentially imparts energy to the duplex that is available to drive DNA transactions, such as transcription, replication and recombination. The topological state of DNA also influences its affinity for some DNA binding proteins, especially in DNA sequences that have a high A + T base content. The underwinding of DNA by the ATP-dependent topoisomerase DNA gyrase creates a continuum between metabolic flux, DNA topology and gene expression that underpins the global response of the genome to changes in the intracellular and external environments. These connections describe a fundamental and generalised mechanism affecting global gene expression that underlies the specific control of transcription operating through conventional transcription factors. This mechanism also provides a basal level of control for genes acquired by horizontal DNA transfer, assisting microbial evolution, including the evolution of pathogenic bacteria.

Transcription-coupled DNA supercoiling dictates the chromosomal arrangement of bacterial genes

Over the recent decade, the central importance of DNA supercoiling in chromosome organization and global gene regulation of bacteria became more and more visible. With a regulon comprising more than 2000 genes in Escherichia coli, DNA supercoiling is among the most influential regulators of gene expression found in bacteria so far. However, the mechanism creating thousands of diverse temporal gene expression patterns coordinated by DNA supercoiling remains unclear. In this study we show that a specific chromosomal arrangement of genes modulates the local levels of DNA supercoiling at gene promoters via transcription-coupled DNA supercoiling (TCDS) in the model organism E. coli. Our findings provide a consistent explanation for the strong positive coupling of temporal gene expression patterns of neighboring genes. Using comparative genomics we are furthermore able to provide evidence that TCDS is a driving force for the evolution of chromosomal gene arrangement patterns in other Enterobacteriaceae. With the currently available data of promoter supercoiling sensitivity we prove that the same principle is applicable also for the evolutionary distant gram-positive pathogenic bacterium Streptococcus pneumoniae. Moreover, our findings are fully consistent with recent investigations concerning the regulatory impact of TCDS on gene pairs in eukaryots underpinning the broad applicability of our analysis.

DNA supercoiling, a critical signal regulating the basal expression of the lac operon in Escherichia coli

Escherichia coli lac repressor (LacI) is a paradigmatic transcriptional factor that controls the expression of lacZYA in the lac operon. This tetrameric protein specifically binds to the O1, O2 and O3 operators of the lac operon and forms a DNA loop to repress transcription from the adjacent lac promoter. In this article, we demonstrate that upon binding to the O1 and O2 operators at their native positions LacI constrains three (−) supercoils within the 401-bp DNA loop of the lac promoter and forms a topological barrier. The stability of LacI-mediated DNA topological barriers is directly proportional to its DNA binding affinity. However, we find that DNA supercoiling modulates the basal expression from the lac operon in E. coli. Our results are consistent with the hypothesis that LacI functions as a topological barrier to constrain free, unconstrained (−) supercoils within the 401-bp DNA loop of the lac promoter. These constrained (−) supercoils enhance LacI’s DNA-binding affinity and thereby the repression of the promoter. Thus, LacI binding is superhelically modulated to control the expression of lacZYA in the lac operon under varying growth conditions.

Chromosome position effects on gene expression in Escherichia coli K-12

In eukaryotes, the location of a gene on the chromosome is known to affect its expression, but such position effects are poorly understood in bacteria. Here, using Escherichia coli K-12, we demonstrate that expression of a reporter gene cassette, comprised of the model E. coli lac promoter driving expression of gfp, varies by ∼300-fold depending on its precise position on the chromosome. At some positions, expression was more than 3-fold higher than at the natural lac promoter locus, whereas at several other locations, the reporter cassette was completely silenced: effectively overriding local lac promoter control. These effects were not due to differences in gene copy number, caused by partially replicated genomes. Rather, the differences in gene expression occur predominantly at the level of transcription and are mediated by several different features that are involved in chromosome organization. Taken together, our findings identify a tier of gene regulation above local promoter control and highlight the importance of chromosome position effects on gene expression profiles in bacteria.

Transcription-coupled DNA supercoiling in defined protein systems and in E. coli topA mutant strains

Transcription by RNA polymerases can stimulate (-) DNA supercoiling both in vitro and in Escherichia coli topA strains. This phenomenon has been successfully explained by a "twin-supercoiled-domain" model of transcription in which (+) supercoils are produced in front of the transcribing RNA polymerase and (-) supercoils behind it. Previously, it has been shown that certain sequence-specific DNA-binding proteins potently stimulate transcription-coupled DNA supercoiling (TCDS) in an in vitro protein system. These results are consistent with a topological barrier model where certain nucleoprotein complexes can form topological barriers that impede the diffusion and merger of independent chromosomal supercoil domains. Indeed, recent biochemical and single-molecule results demonstrated the existence of nucleoprotein-based DNA topological barriers, which are capable of dividing a DNA molecule into different topological domains. Additionally, recent in vivo studies showed that a transcriptional ensemble (including the transcribing RNA polymerase and the RNA transcript) alone is sufficient to cause a change in local DNA superhelicity. This topological change in local chromosome structure should have a great impact on the conformation and function of critical DNA sequence elements, such as promoters and DNA replication origins. In this article, we will also review recent progress by which TCDS is a critical stimulating force to activate transcription initiation from weak promoters, such as the Salmonella typhimurium leu-500 promoter.