The effect of mobile element IS10 on experimental regulatory evolution in Escherichia coli

RpoS and the bacterial general stress response

SUMMARYThe general stress response (GSR) is a widespread strategy developed by bacteria to adapt and respond to their changing environments. The GSR is induced by one or multiple simultaneous stresses, as well as during entry into stationary phase and leads to a global response that protects cells against multiple stresses. The alternative sigma factor RpoS is the central GSR regulator in E. coli and conserved in most γ-proteobacteria. In E. coli, RpoS is induced under conditions of nutrient deprivation and other stresses, primarily via the activation of RpoS translation and inhibition of RpoS proteolysis. This review includes recent advances in our understanding of how stresses lead to RpoS induction and a summary of the recent studies attempting to define RpoS-dependent genes and pathways.

Hypermutator emergence in experimental Escherichia coli populations is stress-type dependent

Genotypes exhibiting an increased mutation rate, called hypermutators, can propagate in microbial populations because they can have an advantage due to the higher supply of beneficial mutations needed for adaptation. Although this is a frequently observed phenomenon in natural and laboratory populations, little is known about the influence of parameters such as the degree of maladaptation, stress intensity, and the genetic architecture for adaptation on the emergence of hypermutators. To address this knowledge gap, we measured the emergence of hypermutators over ~1,000 generations in experimental Escherichia coli populations exposed to different levels of osmotic or antibiotic stress. Our stress types were chosen based on the assumption that the genetic architecture for adaptation differs between them. Indeed, we show that the size of the genetic basis for adaptation is larger for osmotic stress compared to antibiotic stress. During our experiment, we observed an increased emergence of hypermutators in populations exposed to osmotic stress but not in those exposed to antibiotic stress, indicating that hypermutator emergence rates are stress type dependent. These results support our hypothesis that hypermutator emergence is linked to the size of the genetic basis for adaptation. In addition, we identified other parameters that covaried with stress type (stress level and IS transposition rates) that might have contributed to an increased hypermutator provision and selection. Our results provide a first comparison of hypermutator emergence rates under varying stress conditions and point towards complex interactions of multiple stress-related factors on the evolution of mutation rates.

Lanza V, Rodríguez-Beltrán J, Galán JC, San Millán A, Cantón R, Coque TM. Evolutionary Pathways and Trajectories in Antibiotic Resistance

Evolution is the hallmark of life. Descriptions of the evolution of microorganisms have provided a wealth of information, but knowledge regarding "what happened" has precluded a deeper understanding of "how" evolution has proceeded, as in the case of antimicrobial resistance. The difficulty in answering the "how" question lies in the multihierarchical dimensions of evolutionary processes, nested in complex networks, encompassing all units of selection, from genes to communities and ecosystems. At the simplest ontological level (as resistance genes), evolution proceeds by random (mutation and drift) and directional (natural selection) processes; however, sequential pathways of adaptive variation can occasionally be observed, and under fixed circumstances (particular fitness landscapes), evolution is predictable. At the highest level (such as that of plasmids, clones, species, microbiotas), the systems' degrees of freedom increase dramatically, related to the variable dispersal, fragmentation, relatedness, or coalescence of bacterial populations, depending on heterogeneous and changing niches and selective gradients in complex environments. Evolutionary trajectories of antibiotic resistance find their way in these changing landscapes subjected to random variations, becoming highly entropic and therefore unpredictable. However, experimental, phylogenetic, and ecogenetic analyses reveal preferential frequented paths (highways) where antibiotic resistance flows and propagates, allowing some understanding of evolutionary dynamics, modeling and designing interventions. Studies on antibiotic resistance have an applied aspect in improving individual health, One Health, and Global Health, as well as an academic value for understanding evolution. Most importantly, they have a heuristic significance as a model to reduce the negative influence of anthropogenic effects on the environment.

Transposable Element Mobilization in Interspecific Yeast Hybrids

Barbara McClintock first hypothesized that interspecific hybridization could provide a “genomic shock” that leads to the mobilization of transposable elements (TEs). This hypothesis is based on the idea that regulation of TE movement is potentially disrupted in hybrids. However, the handful of studies testing this hypothesis have yielded mixed results. Here, we set out to identify if hybridization can increase transposition rate and facilitate colonization of TEs in Saccharomyces cerevisiae × Saccharomyces uvarum interspecific yeast hybrids. Saccharomyces cerevisiae have a small number of active long terminal repeat retrotransposons (Ty elements), whereas their distant relative S. uvarum have lost the Ty elements active in S. cerevisiae. Although the regulation system of Ty elements is known in S. cerevisiae, it is unclear how Ty elements are regulated in other Saccharomyces species, and what mechanisms contributed to the loss of most classes of Ty elements in S. uvarum. Therefore, we first assessed whether TEs could insert in the S. uvarum sub-genome of a S. cerevisiae × S. uvarum hybrid. We induced transposition to occur in these hybrids and developed a sequencing technique to show that Ty elements insert readily and nonrandomly in the S. uvarum genome. We then used an in vivo reporter construct to directly measure transposition rate in hybrids, demonstrating that hybridization itself does not alter rate of mobilization. However, we surprisingly show that species-specific mitochondrial inheritance can change transposition rate by an order of magnitude. Overall, our results provide evidence that hybridization can potentially facilitate the introduction of TEs across species boundaries and alter transposition via mitochondrial transmission, but that this does not lead to unrestrained proliferation of TEs suggested by the genomic shock theory.

Regulatory molecule cAMP changes cell fitness of the engineered Escherichia coli for terpenoids production

Terpenoids are a class of natural compounds with many important functions and applications. They are synthesized from a long synthetic pathway of isoprenyl unit coupling with the myriads of terpene synthases. Owing to the catalytic divergence of terpenoids synthesis, microbial production of terpenoids is compromised to the complexity of pathway engineering and suffers from the metabolic engineering burden. In this work, the adaptive Escherichia coli HP variant exhibited a general cell fitness in terpenoid synthesis. Especially, it could yield taxadiene of 193.2 mg/L in a test tube culture, which is a five-fold increase over the production in the wild type E. coli DH5α. Mutational analyses indicated that IS10 insertion in adenylate cyclase CyaA (CyaA_HP) resulted in lowering intracellular cyclic AMP (cAMP), which could regulate its receptor protein CRP to rewire cell metabolism and contributed to the improved cell fitness. Our results suggested a way to manipulate cell fitness for terpenoids production and other products.

A glimpse of potential transposable element impact on adaptation of the industrial yeast Saccharomyces cerevisiae

The adapted industrial yeast strain Saccharomyces cerevisiae NRRL Y-50049 is able to in situ detoxify major toxic aldehyde compounds derived from sugar conversion of lignocellulosic biomass while producing ethanol. Pathway-based studies on its mechanisms of tolerance have been reported previously, however, little is known about transposable element (TE) involvement in its adaptation to inhibitory compounds. This work presents a comparative dynamic transcription expression analysis in response to a toxic treatment between Y-50049 and its progenitor, an industrial type strain NRRL Y-12632, using a time-course study. At least 77 TEs from Y-50049 showed significantly increased expression compared with its progenitor, especially during the late lag phase. Sequence analysis revealed significant differences in TE sequences between the two strains. Y-50049 was also found to have a transposons of yeast 2 (Ty2) long terminal repeat-linked YAT1 gene showing significantly higher copy number changes than its progenitor. These results raise awareness of potential TE involvement in the adaptation of industrial yeast to the tolerance of toxic chemicals.

Evolutionary Repair Experiments as a Window to the Molecular Diversity of Life

Comparative genomics reveals an unexpected diversity in the molecular mechanisms underlying conserved cellular functions, such as DNA replication and cytokinesis. However, the genetic bases and evolutionary processes underlying this 'molecular diversity' remain to be explained. Here, we review a tool to generate alternative mechanisms for conserved cellular functions and test hypotheses concerning the generation of molecular diversity - evolutionary repair experiments, in which laboratory microbial populations adapt in response to a genetic perturbation. We summarize the insights gained from evolutionary repair experiments, the spectrum and dynamics of compensatory mutations, and the alternative molecular mechanisms used to repair perturbed cellular functions. We relate these experiments to the modifications of conserved functions that have occurred outside the laboratory. We end by proposing strategies to improve evolutionary repair experiments as a tool to explore the molecular diversity of life.

Plasticity of Promoter-Core Sequences Allows Bacteria to Compensate for the Loss of a Key Global Regulatory Gene

Transcription regulatory networks (TRNs) are of central importance for both short-term phenotypic adaptation in response to environmental fluctuations and long-term evolutionary adaptation, with global regulatory genes often being targets of natural selection in laboratory experiments. Here, we combined evolution experiments, whole-genome resequencing, and molecular genetics to investigate the driving forces, genetic constraints, and molecular mechanisms that dictate how bacteria can cope with a drastic perturbation of their TRNs. The crp gene, encoding a major global regulator in Escherichia coli, was deleted in four different genetic backgrounds, all derived from the Long-Term Evolution Experiment (LTEE) but with different TRN architectures. We confirmed that crp deletion had a more deleterious effect on growth rate in the LTEE-adapted genotypes; and we showed that the ptsG gene, which encodes the major glucose-PTS transporter, gained CRP (cyclic AMP receptor protein) dependence over time in the LTEE. We then further evolved the four crp-deleted genotypes in glucose minimal medium, and we found that they all quickly recovered from their growth defects by increasing glucose uptake. We showed that this recovery was specific to the selective environment and consistently relied on mutations in the cis-regulatory region of ptsG, regardless of the initial genotype. These mutations affected the interplay of transcription factors acting at the promoters, changed the intrinsic properties of the existing promoters, or produced new transcription initiation sites. Therefore, the plasticity of even a single promoter region can compensate by three different mechanisms for the loss of a key regulatory hub in the E. coli TRN.

Evolutionary Changes after Translational Challenges Imposed by Horizontal Gene Transfer

Genes acquired by horizontal gene transfer (HGT) may provide the recipient organism with potentially new functions, but proper expression level and integration of the transferred genes in the novel environment are not granted. Notably, transferred genes can differ from the receiving genome in codon usage preferences, leading to impaired translation and reduced functionality. Here, we characterize the genomic and proteomic changes undergone during experimental evolution of Escherichia coli after HGT of three synonymous versions, presenting very different codon usage preference, of an antibiotic resistance gene. The experimental evolution was conducted with and without the corresponding antibiotic and the mutational patterns and proteomic profiles after 1,000 generations largely depend on the experimental growth conditions (e.g., mutations in antibiotic off-target genes), and on the synonymous gene version transferred (e.g., mutations in genes responsive to translational stress). The transfer of an exogenous gene extensively modifies the whole proteome, and these proteomic changes are different for the different version of the transferred gene. Additionally, we identified conspicuous changes in global regulators and in intermediate metabolism, confirmed the evolutionary ratchet generated by mutations in DNA repair genes and highlighted the plasticity of bacterial genomes accumulating large and occasionally transient duplications. Our results support a central role of HGT in fuelling evolution as a powerful mechanism promoting rapid, often dramatic genotypic and phenotypic changes. The profound reshaping of the pre-existing geno/phenotype allows the recipient bacteria to explore new ways of functioning, far beyond the mere acquisition of a novel function.

Insertion sequence elements-mediated structural variations in bacterial genomes

Mobile genetic elements (MGEs) impact the evolution and stability of their host genomes. Insertion sequence (IS) elements are the most common MGEs in bacterial genomes and play a crucial role in mediating large-scale variations in bacterial genomes. It is understood that IS elements and MGEs in general coexist in a dynamical equilibrium with their respective hosts. Current studies indicate that the spontaneous movement of IS elements does not follow a constant rate in different bacterial genomes. However, due to the paucity and sparsity of the data, these observations are yet to be conclusive. In this paper, we conducted a comparative analysis of the IS-mediated genome structural variations in ten mutation accumulation (MA) experiments across eight strains of five bacterial species containing IS elements, including four strains of the E. coli. We used GRASPER algorithm, a denovo structural variation (SV) identification algorithm designed to detect SVs involving repetitive sequences in the genome. We observed highly diverse rates of IS insertions and IS-mediated recombinations across different bacterial species as well as across different strains of the same bacterial species. We also observed different rates of the elements from the same IS family in different bacterial genomes, suggesting that the distinction in rates might not be due to the different composition of IS elements across bacterial genomes.

Instability of the 16S rRNA methyltransferase-encoding npmA gene: why have bacterial cells possessing npmA not spread despite their high and broad resistance to aminoglycosides?

The NpmA bacterial 16S rRNA methyltransferase, which is identified from Escherichia coli strains, confers high resistance to many types of aminoglycoside upon its host cells. But despite its resistance-conferring ability, only two cases of its isolation from E. coli (14 years apart) have been reported to date. Here, we investigated the effect of the npmA gene on aminoglycoside resistance in Pseudomonas aeruginosa and Klebsiella pneumoniae and its stability in E. coli cells by comparing it with armA, another 16S rRNA methyltransferase gene currently spreading globally. As a result, we found that npmA conferred resistance to all types of aminoglycoside antibiotics we tested (except streptomycin) in both P. aeruginosa and K. pneumoniae, as well in E. coli. In addition, co-expression of armA and npmA resulted in an additive effect for the resistance. However, in return for the resistance, we also observed that the growth rates and the cell survivability of the strains transformed with the npmA-harboring plasmids were inferior than those of the control strains and that these plasmids were easily disrupted by IS10, IS1, and IS5 insertion sequences. We discuss these data in the context of the threat posed by pathogenic strains possessing npmA.

Experimental evolution of diverse Escherichia coli metabolic mutants identifies genetic loci for convergent adaptation of growth rate

Cell growth is determined by substrate availability and the cell’s metabolic capacity to assimilate substrates into building blocks. Metabolic genes that determine growth rate may interact synergistically or antagonistically, and can accelerate or slow growth, depending on genetic background and environmental conditions. We evolved a diverse set of Escherichia coli single-gene deletion mutants with a spectrum of growth rates and identified mutations that generally increase growth rate. Despite the metabolic differences between parent strains, mutations that enhanced growth largely mapped to core transcription machinery, including the β and β’ subunits of RNA polymerase (RNAP) and the transcription elongation factor, NusA. The structural segments of RNAP that determine enhanced growth have been previously implicated in antibiotic resistance and in the control of transcription elongation and pausing. We further developed a computational framework to characterize how the transcriptional changes that occur upon acquisition of these mutations affect growth rate across strains. Our experimental and computational results provide evidence for cases in which RNAP mutations shift the competitive balance between active transcription and gene silencing. This study demonstrates that mutations in specific regions of RNAP are a convergent adaptive solution that can enhance the growth rate of cells from distinct metabolic states.

The loss of a metabolic function caused by gene deletion can be compensated, in certain cases, by the concurrent mutation of a second gene. Whether such gene pairs share a local chemical or regulatory relationship or interact via a non-local mechanism has implications for the co-evolution of genetic changes, development of alternatives to gene therapy, and the design of combination antimicrobial therapies that select against resistance. Yet, we lack a comprehensive knowledge of adaptive responses to metabolic mutations, and our understanding of the mechanisms underlying genetic rescue remains limited. We present results of a laboratory evolution approach that has the potential to address both challenges, showing that mutations in specific regions of RNA polymerase enhance growth rates of distinct mutant strains of Escherichia coli with a spectrum of growth defects. Several of these adaptive mutations are deleterious when engineered directly into the original wild-type strain under alternative cultivation conditions, and thus have epistatic rescue properties when paired with the corresponding primary metabolic gene deletions. Our combination of adaptive evolution, directed genetic engineering, and mathematical analysis of transcription and growth rate distinguishes between rescue interactions that are specific or non-specific to a particular deletion. Our study further supports a model for RNA polymerase as a locus of convergent adaptive evolution from different sub-optimal metabolic starting points.

Mammalian transposable elements and their impacts on genome evolution

Transposable elements (TEs) are genetic elements with the ability to mobilize and replicate themselves in a genome. Mammalian genomes are dominated by TEs, which can reach copy numbers in the hundreds of thousands. As a result, TEs have had significant impacts on mammalian evolution. Here we summarize the current understanding of TE content in mammal genomes and find that, with a few exceptions, most fall within a predictable range of observations. First, one third to one half of the genome is derived from TEs. Second, most mammalian genomes are dominated by LINE and SINE retrotransposons, more limited LTR retrotransposons, and minimal DNA transposon accumulation. Third, most mammal genome contains at least one family of actively accumulating retrotransposon. Finally, horizontal transfer of TEs among lineages is rare. TE exaptation events are being recognized with increasing frequency. Despite these beneficial aspects of TE content and activity, the majority of TE insertions are neutral or deleterious. To limit the deleterious effects of TE proliferation, the genome has evolved several defense mechanisms that act at the epigenetic, transcriptional, and post-transcriptional levels. The interaction between TEs and these defense mechanisms has led to an evolutionary arms race where TEs are suppressed, evolve to escape suppression, then are suppressed again as the defense mechanisms undergo compensatory change. The result is complex and constantly evolving interactions between TEs and host genomes.

Regulation of solvent tolerance in Pseudomonas putida S12 mediated by mobile elements

Organic solvent‐tolerant bacteria are outstanding and versatile hosts for the bio‐based production of a broad range of generally toxic aromatic compounds. The energetically costly solvent tolerance mechanisms are subject to multiple levels of regulation, involving among other mobile genetic elements. The genome of the solvent‐tolerant Pseudomonas putida S12 contains many such mobile elements that play a major role in the regulation and adaptation to various stress conditions, including the regulation of expression of the solvent efflux pump SrpABC. We recently sequenced the genome of P. putida S12. Detailed annotation identified a threefold higher copy number of the mobile element ISS12 in contrast to earlier observations. In this study, we describe the mobile genetic elements and elaborate on the role of ISS12 in the establishment and maintenance of solvent tolerance in P. putida. We identified three different variants of ISS12 of which a single variant exhibits a high translocation rate. One copy of this variant caused a loss of solvent tolerance in the sequenced strain by disruption of srpA. Solvent tolerance could be restored by applying selective pressure, leading to a clean excision of the mobile element.

DNA Transposition at Work

DNA transposons are defined segments of DNA that are able to move from one genomic location to another. Movement is facilitated by one or more proteins, called the transposase, typically encoded by the mobile element itself. Here, we first provide an overview of the classification of such mobile elements in a variety of organisms. From a mechanistic perspective, we have focused on one particular group of DNA transposons that encode a transposase with a DD(E/D) catalytic domain that is topologically similar to RNase H. For these, a number of three-dimensional structures of transpososomes (transposase-nucleic acid complexes) are available, and we use these to describe the basics of their mechanisms. The DD(E/D) group, in addition to being the largest and most common among all DNA transposases, is the one whose members have been used for a wide variety of genomic applications. Therefore, a second focus of the article is to provide a nonexhaustive overview of transposon applications. Although several non-transposon-based approaches to site-directed genome modifications have emerged in the past decade, transposon-based applications are highly relevant when integration specificity is not sought. In fact, for many applications, the almost-perfect randomness and high frequency of integration make transposon-based approaches indispensable.

The dynamics of mobile promoters: Enhanced stability in promoter regions

Mobile promoters are emerging as a new class of mobile genetic elements, first identified by examining prokaryote genome sequences, and more recently confirmed by experimental observations in bacteria. Recent datasets have identified over 40,000 putative mobile promoters in sequenced prokaryote genomes, however only one-third of these are in regions of the genome directly upstream from coding sequences, that is, in promoter regions. The presence of many promoter sequences in non-promoter regions is unexplained. Here we develop a general mathematical model for the dynamics of mobile promoters, extending previous work to capture the dynamics both within and outside promoter regions. From this general model, we apply rigorous model selection techniques to identify which parameters are statistically justified in describing the available mobile promoter data, and find best-fit values of these parameters. Our results suggest that high rates of horizontal gene transfer maintain the population of mobile promoters in promoter regions, and that once established at these sites, mobile promoters are rarely lost, but are commonly copied to other genomic regions. In contrast, mobile promoter copies in non-promoter regions are more numerous and more volatile, experiencing substantially higher rates of duplication, loss and diversification.

Adaptation to Parasites and Costs of Parasite Resistance in Mutator and Nonmutator Bacteria

Parasitism creates selection for resistance mechanisms in host populations and is hypothesized to promote increased host evolvability. However, the influence of these traits on host evolution when parasites are no longer present is unclear. We used experimental evolution and whole-genome sequencing of Escherichia coli to determine the effects of past and present exposure to parasitic viruses (phages) on the spread of mutator alleles, resistance, and bacterial competitive fitness. We found that mutator alleles spread rapidly during adaptation to any of four different phage species, and this pattern was even more pronounced with multiple phages present simultaneously. However, hypermutability did not detectably accelerate adaptation in the absence of phages and recovery of fitness costs associated with resistance. Several lineages evolved phage resistance through elevated mucoidy, and during subsequent evolution in phage-free conditions they rapidly reverted to nonmucoid, phage-susceptible phenotypes. Genome sequencing revealed that this phenotypic reversion was achieved by additional genetic changes rather than by genotypic reversion of the initial resistance mutations. Insertion sequence (IS) elements played a key role in both the acquisition of resistance and adaptation in the absence of parasites; unlike single nucleotide polymorphisms, IS insertions were not more frequent in mutator lineages. Our results provide a genetic explanation for rapid reversion of mucoidy, a phenotype observed in other bacterial species including human pathogens. Moreover, this demonstrates that the types of genetic change underlying adaptation to fitness costs, and consequently the impact of evolvability mechanisms such as increased point-mutation rates, depend critically on the mechanism of resistance.

Dynamics of bacterial insertion sequences: can transposition bursts help the elements persist?

Background

Currently there is no satisfactory explanation for why bacterial insertion sequences (ISs) widely occur across prokaryotes despite being mostly harmful to their host genomes. Rates of horizontal gene transfer are likely to be too low to maintain ISs within a population. IS-induced beneficial mutations may be important for both prevalence of ISs and microbial adaptation to changing environments but may be too rare to sustain IS elements in the long run. Environmental stress can induce elevated rates of IS transposition activities; such episodes are known as ‘transposition bursts’. By examining how selective forces and transposition events interact to influence IS dynamics, this study asks whether transposition bursts can lead to IS persistence.

Results

We show through a simulation model that ISs are gradually eliminated from a population even if IS transpositions occasionally cause advantageous mutations. With beneficial mutations, transposition bursts create variation in IS copy numbers and improve cell fitness on average. However, these benefits are not usually sufficient to overcome the negative selection against the elements, and transposition bursts amplify the mean fitness effect which, if negative, simply accelerates the extinction of ISs. If down regulation of transposition occurs, IS extinctions are reduced while ISs still generate variation amongst bacterial genomes.

Conclusions

Transposition bursts do not help ISs persist in a bacterial population in the long run because most burst-induced mutations are deleterious and therefore not favoured by natural selection. However, bursts do create more genetic variation through which occasional advantageous mutations can help organisms adapt. Regulation of IS transposition bursts and stronger positive selection of the elements interact to slow down the burst-induced extinction of ISs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0560-5) contains supplementary material, which is available to authorized users.

A new practical guide to the Luria-Delbrück protocol

Since 2000 several review papers have been published about the analysis of experimental data obtained using the Luria-Delbrück protocol. These timely papers cleared much of the confusion surrounding various methods for estimating or comparing mutation rates. As a result, today the fluctuation test is more widely applied with much improved accuracy. The present paper provides guidelines on a few remaining problems that continue to baffle mutation researchers. Among the issues addressed are incomplete plating, relative fitness, and comparison of experiments where average final cell population sizes differ. It also offers a fresh view on the estimation methods that are based on the sample median.

Extinction probabilities and stationary distributions of mobile genetic elements in prokaryotes: The birth-death-diversification model

Theoretical approaches are essential to our understanding of the complex dynamics of mobile genetic elements (MGEs) within genomes. Recently, the birth-death-diversification model was developed to describe the dynamics of mobile promoters (MPs), a particular class of MGEs in prokaryotes. A unique feature of this model is that genetic diversification of elements was included. To explore the implications of diversification on the longterm fate of MGE lineages, in this contribution we analyze the extinction probabilities, extinction times and equilibrium solutions of the birth-death-diversification model. We find that diversification increases both the survival and growth rate of MGE families, but the strength of this effect depends on the rate of horizontal gene transfer (HGT). We also find that the distribution of MGE families per genome is not necessarily monotonically decreasing, as observed for MPs, but may have a peak in the distribution that is related to the HGT rate. For MPs specifically, we find that new families have a high extinction probability, and predict that the number of MPs is increasing, albeit at a very slow rate. Additionally, we develop an extension of the birth-death-diversification model which allows MGEs in different regions of the genome, for example coding and non-coding, to be described by different rates. This extension may offer a potential explanation as to why the majority of MPs are located in non-promoter regions of the genome.

Evolutionary Trends of the Transposase-Encoding Open Reading Frames A and B (orfA and orfB) of the Mycobacterial IS6110 Insertion Sequence

Background

The IS6110 insertion sequence, a member of the IS3 family of insertion sequences, was found to be specific to the Mycobacterium tuberculosis complex (MTBC). Although IS6110 has been extensively characterized as a transposable genetic marker, the evolutionary history of its own transposase-encoding sequence has not, to the best of our knowledge, been investigated.

Methodology/Principal Findings

Here we explored the evolution of the IS6110 sequence by analysing the genetic variability and the selective forces acting on its transposase-encoding open reading frames (ORFs) A and B (orfA and orfB). For this purpose, we used a strain collection consisting of smooth tubercle bacilli (STB), an early branching lineage of the MTBC, and present-day M. tuberculosis strains representing the full breadth of genetic diversity in Tunisia. In each ORF, we found a major haplotype that dominated over a flat distribution of rare descendent haplotypes, consisting mainly of single- and double-nucleotide variant singletons. The predominant haplotypes consisted of both ancestral and present-day strains, suggesting that IS6110 acquisition predated the emergence of the MTBC. There was no evidence of recombination and both ORFs were subjected to strict purifying selection, as demonstrated by their dN/dS ratios (0.29 and 0.51, respectively), as well as their significantly negative Tajima’s D statistics. Strikingly, the purifying selection acting on orfA proved much more stringent, suggesting its critical role in regulating the transpositional process. Maximum likelihood analyses further excluded any possibility of positive selection acting on single amino acid residues.

Conclusions/Significance

Taken together our data fit with an evolutionary scenario according to which the observed variability pattern of the IS6110 transposase-encoding ORFs is generated mainly through random point mutations that accrued on a functionally optimal IS6110 copy, whose acquisition predated the emergence of the MTBC complex. Background selection acting against deleterious mutations led to an excess of low-frequency variants.

From passengers to drivers: Impact of bacterial transposable elements on evolvability

Microbes have several mechanisms that promote evolutionary adaptation in stressful environments. The corresponding molecular pathways promote diversity through modulating rates of recombination, mutation or influence the activity of transposable genetic elements. Recent experimental studies suggest an evolutionary conflict between these mechanisms. Specifically, presence of mismatch repair mutator alleles in a bacterial population dramatically reduced fixation of bacterial insertion sequence elements. When rare, these elements had only a limited impact on adaptive evolution compared with other mutation-generating pathways. IS elements may initially spread like molecular parasites, but once present in many copies in a given genome, they might become generators of novelty during bacterial evolution.

Genomic parasites or symbionts? Modeling the effects of environmental pressure on transposition activity in asexual populations

Transposable elements are DNA segments capable of persisting in host genomes by self-replication in spite of deleterious mutagenic effects. The theoretical dynamics of these elements within genomes has been studied extensively, and population genetic models predict that they can invade and maintain as a result of both intra-genomic and inter-individual selection in sexual species. In asexuals, the success of selfish DNA is more difficult to explain. However, most theoretical work assumes constant environment. Here, we analyze the impact of environmental change on the dynamics of transposition activity when horizontal DNA exchange is absent, based on a stochastic computational model of transposable element proliferation. We argue that repeated changes in the phenotypic optimum in a multidimensional fitness landscape may induce explosive bursts of transposition activity associated with faster adaptation. However, long-term maintenance of transposition activity is unlikely. This could contribute to the significant variation in the transposable element copy number among closely related species.

Promoter propagation in prokaryotes

Transcriptional activation or ‘rewiring’ of silent genes is an important, yet poorly understood, phenomenon in prokaryotic genomes. Anecdotal evidence coming from experimental evolution studies in bacterial systems has shown the promptness of adaptation upon appropriate selective pressure. In many cases, a partial or complete promoter is mobilized to silent genes from elsewhere in the genome. We term hereafter such recruited regulatory sequences as Putative Mobile Promoters (PMPs) and we hypothesize they have a large impact on rapid adaptation of novel or cryptic functions. Querying all publicly available prokaryotic genomes (1362) uncovered >4000 families of highly conserved PMPs (50 to 100 long with ≥80% nt identity) in 1043 genomes from 424 different genera. The genomes with the largest number of PMP families are Anabaena variabilis (28 families), Geobacter uraniireducens (27 families) and Cyanothece PCC7424 (25 families). Family size varied from 2 to 93 homologous promoters (in Desulfurivibrio alkaliphilus). Some PMPs are present in particular species, but some are conserved across distant genera. The identified PMPs represent a conservative dataset of very recent or conserved events of mobilization of non-coding DNA and thus they constitute evidence of an extensive reservoir of recyclable regulatory sequences for rapid transcriptional rewiring.

Competition between transposable elements and mutator genes in bacteria

Although both genotypes with elevated mutation rate (mutators) and mobilization of insertion sequence (IS) elements have substantial impact on genome diversification, their potential interactions are unknown. Moreover, the evolutionary forces driving gradual accumulation of these elements are unclear: Do these elements spread in an initially transposon-free bacterial genome as they enable rapid adaptive evolution? To address these issues, we inserted an active IS1 element into a reduced Escherichia coli genome devoid of all other mobile DNA. Evolutionary laboratory experiments revealed that IS elements increase mutational supply and occasionally generate variants with especially large phenotypic effects. However, their impact on adaptive evolution is small compared with mismatch repair mutator alleles, and hence, the latter impede the spread of IS-carrying strains. Given their ubiquity in natural populations, such mutator alleles could limit early phase of IS element evolution in a new bacterial host. More generally, our work demonstrates the existence of an evolutionary conflict between mutation-promoting mechanisms.

Adaptive evolution of the lactose utilization network in experimentally evolved populations of Escherichia coli

Adaptation to novel environments is often associated with changes in gene regulation. Nevertheless, few studies have been able both to identify the genetic basis of changes in regulation and to demonstrate why these changes are beneficial. To this end, we have focused on understanding both how and why the lactose utilization network has evolved in replicate populations of Escherichia coli. We found that lac operon regulation became strikingly variable, including changes in the mode of environmental response (bimodal, graded, and constitutive), sensitivity to inducer concentration, and maximum expression level. In addition, some classes of regulatory change were enriched in specific selective environments. Sequencing of evolved clones, combined with reconstruction of individual mutations in the ancestral background, identified mutations within the lac operon that recapitulate many of the evolved regulatory changes. These mutations conferred fitness benefits in environments containing lactose, indicating that the regulatory changes are adaptive. The same mutations conferred different fitness effects when present in an evolved clone, indicating that interactions between the lac operon and other evolved mutations also contribute to fitness. Similarly, changes in lac regulation not explained by lac operon mutations also point to important interactions with other evolved mutations. Together these results underline how dynamic regulatory interactions can be, in this case evolving through mutations both within and external to the canonical lactose utilization network.

Optical mapping and sequencing of the Escherichia coli KO11 genome reveal extensive chromosomal rearrangements, and multiple tandem copies of the Zymomonas mobilis pdc and adhB genes

Escherichia coli KO11 (ATCC 55124) was engineered in 1990 to produce ethanol by chromosomal insertion of the Zymomonas mobilis pdc and adhB genes into E. coli W (ATCC 9637). KO11FL, our current laboratory version of KO11, and its parent E. coli W were sequenced, and contigs assembled into genomic sequences using optical NcoI restriction maps as templates. E. coli W contained plasmids pRK1 (102.5 kb) and pRK2 (5.4 kb), but KO11FL only contained pRK2. KO11FL optical maps made with AflII and with BamHI showed a tandem repeat region, consisting of at least 20 copies of a 10-kb unit. The repeat region was located at the insertion site for the pdc, adhB, and chloramphenicol-resistance genes. Sequence coverage of these genes was about 25-fold higher than average, consistent with amplification of the foreign genes that were inserted as circularized DNA. Selection for higher levels of chloramphenicol resistance originally produced strains with higher pdc and adhB expression, and hence improved fermentation performance, by increasing the gene copy number. Sequence data for an earlier version of KO11, ATCC 55124, indicated that multiple copies of pdc adhB were present. Comparison of the W and KO11FL genomes showed large inversions and deletions in KO11FL, mostly enabled by IS10, which is absent from W but present at 30 sites in KO11FL. The early KO11 strain ATCC 55124 had no rearrangements, contained only one IS10, and lacked most accumulated single nucleotide polymorphisms (SNPs) present in KO11FL. Despite rearrangements and SNPs in KO11FL, fermentation performance was equal to that of ATCC 55124.