Bacterial interspersed mosaic elements (BIMEs) are a major source of sequence polymorphism in Escherichia coli intergenic regions including specific associations with a new insertion sequence

Synthetic 3'-UTR valves for optimal metabolic flux control in Escherichia coli

As the design of genetic circuitry for synthetic biology becomes more sophisticated, diverse regulatory bioparts are required. Despite their importance, well-characterized 3′-untranslated region (3′-UTR) bioparts are limited. Thus, transcript 3′-ends require further investigation to understand the underlying regulatory role and applications of the 3′-UTR. Here, we revisited the use of Term-Seq in the Escherichia coli strain K-12 MG1655 to enhance our understanding of 3′-UTR regulatory functions and to provide a diverse collection of tunable 3′-UTR bioparts with a wide termination strength range. Comprehensive analysis of 1,629 transcript 3′-end positions revealed multiple 3′-termini classes generated through transcription termination and RNA processing. The examination of individual Rho-independent terminators revealed a reduction in downstream gene expression over a wide range, which led to the design of novel synthetic metabolic valves that control metabolic fluxes in branched pathways. These synthetic metabolic valves determine the optimal balance of heterologous pathways for maximum target biochemical productivity. The regulatory strategy using 3′-UTR bioparts is advantageous over promoter- or 5′-UTR-based transcriptional control as it modulates gene expression at transcription levels without trans-acting element requirements (e.g. transcription factors). Our results provide a foundational platform for 3′-UTR engineering in synthetic biology applications.

How sequence populations persist inside bacterial genomes

Compared to their eukaryotic counterparts, bacterial genomes are small and contain extremely tightly packed genes. Repetitive sequences are rare but not completely absent. One of the most common repeat families is REPINs. REPINs can replicate in the host genome and form populations that persist for millions of years. Here, we model the interactions of these intragenomic sequence populations with the bacterial host. We first confirm well-established results, in the presence and absence of horizontal gene transfer (hgt) sequence populations either expand until they drive the host to extinction or the sequence population gets purged from the genome. We then show that a sequence population can be stably maintained, when each individual sequence provides a benefit that decreases with increasing sequence population size. Maintaining a sequence population of stable size also requires the replication of the sequence population to be costly to the host, otherwise the sequence population size will increase indefinitely. Surprisingly, in regimes with high hgt rates, the benefit conferred by the sequence population does not have to exceed the damage it causes to its host. Our analyses provide a plausible scenario for the persistence of sequence populations in bacterial genomes. We also hypothesize a limited biologically relevant parameter range for the provided benefit, which can be tested in future experiments.

Supercoil Levels in E. coli and Salmonella Chromosomes Are Regulated by the C-Terminal 35⁻38 Amino Acids of GyrA

Prokaryotes have an essential gene-gyrase-that catalyzes negative supercoiling of plasmid and chromosomal DNA. Negative supercoils influence DNA replication, transcription, homologous recombination, site-specific recombination, genetic transposition and sister chromosome segregation. Although E. coli and Salmonella Typhimurium are close relatives with a conserved set of essential genes, E. coli DNA has a supercoil density 15% higher than Salmonella, and E. coli cannot grow at the supercoil density maintained by wild type (WT) Salmonella. E. coli is addicted to high supercoiling levels for efficient chromosomal folding. In vitro experiments were performed with four gyrase isoforms of the tetrameric enzyme (GyrA₂:GyrB₂). E. coli gyrase was more processive and faster than the Salmonella enzyme, but Salmonella strains with chromosomal swaps of E. coli GyrA lost 40% of the chromosomal supercoil density. Reciprocal experiments in E. coli showed chromosomal dysfunction for strains harboring Salmonella GyrA. One GyrA segment responsible for dis-regulation was uncovered by constructing and testing GyrA chimeras in vivo. The six pinwheel elements and the C-terminal 35⁻38 acidic residues of GyrA controlled WT chromosome-wide supercoiling density in both species. A model of enzyme processivity modulated by competition between DNA and the GyrA acidic tail for access to β-pinwheel elements is presented.

Mathematical modelling of CRISPR-Cas system effects on biofilm formation

Clustered regularly interspaced short palindromic repeats (CRISPR), linked with CRISPR associated (Cas) genes, can confer adaptive immunity to bacteria, against bacteriophage infections. Thus from a therapeutic standpoint, CRISPR immunity increases biofilm resistance to phage therapy. Recently, however, CRISPR-Cas genes have been implicated in reducing biofilm formation in lysogenized cells. Thus CRISPR immunity can have complex effects on phage-host-lysogen interactions, particularly in a biofilm. In this contribution, we develop and analyse a series of dynamical systems to elucidate and disentangle these interactions. Two competition models are used to study the effects of lysogens (first model) and CRISPR-immune bacteria (second model) in the biofilm. In the third model, the effect of delivering lysogens to a CRISPR-immune biofilm is investigated. Using standard analyses of equilibria, stability and bifurcations, our models predict that lysogens may be able to displace CRISPR-immune bacteria in a biofilm, and thus suggest strategies to eliminate phage-resistant biofilms.

Bacterial genome instability

Bacterial genomes are remarkably stable from one generation to the next but are plastic on an evolutionary time scale, substantially shaped by horizontal gene transfer, genome rearrangement, and the activities of mobile DNA elements. This implies the existence of a delicate balance between the maintenance of genome stability and the tolerance of genome instability. In this review, we describe the specialized genetic elements and the endogenous processes that contribute to genome instability. We then discuss the consequences of genome instability at the physiological level, where cells have harnessed instability to mediate phase and antigenic variation, and at the evolutionary level, where horizontal gene transfer has played an important role. Indeed, this ability to share DNA sequences has played a major part in the evolution of life on Earth. The evolutionary plasticity of bacterial genomes, coupled with the vast numbers of bacteria on the planet, substantially limits our ability to control disease.

Multiple pathways of duplication formation with and without recombination (RecA) in Salmonella enterica

Duplications are often attributed to "unequal recombination" between separated, directly repeated sequence elements (>100 bp), events that leave a recombinant element at the duplication junction. However, in the bacterial chromosome, duplications form at high rates (10(-3)-10(-5)/cell/division) even without recombination (RecA). Here we describe 1800 spontaneous lac duplications trapped nonselectively on the low-copy F'(128) plasmid, where lac is flanked by direct repeats of the transposable element IS3 (1258 bp) and by numerous quasipalindromic REP elements (30 bp). Duplications form at a high rate (10(-4)/cell/division) that is reduced only about 11-fold in the absence of RecA. With and without RecA, most duplications arise by recombination between IS3 elements (97%). Formation of these duplications is stimulated by IS3 transposase (Tnp) and plasmid transfer functions (TraI). Three duplication pathways are proposed. First, plasmid dimers form at a high rate stimulated by RecA and are then modified by deletions between IS3 elements (resolution) that leave a monomeric plasmid with an IS3-flanked lac duplication. Second, without RecA, duplications occur by single-strand annealing of DNA ends generated in different sister chromosomes after transposase nicks DNA near participating IS3 elements. The absence of RecA may stimulate annealing by allowing chromosome breaks to persist. Third, a minority of lac duplications (3%) have short (0-36 bp) junction sequences (SJ), some of which are located within REP elements. These duplication types form without RecA, Tnp, or Tra by a pathway in which the palindromic junctions of a tandem inversion duplication (TID) may stimulate deletions that leave the final duplication.

Variation in the genomic locations and sequence conservation of STAR elements among staphylococcal species provides insight into DNA repeat evolution

Background

Staphylococcus aureus Repeat (STAR) elements are a type of interspersed intergenic direct repeat. In this study the conservation and variation in these elements was explored by bioinformatic analyses of published staphylococcal genome sequences and through sequencing of specific STAR element loci from a large set of S. aureus isolates.

Results

Using bioinformatic analyses, we found that the STAR elements were located in different genomic loci within each staphylococcal species. There was no correlation between the number of STAR elements in each genome and the evolutionary relatedness of staphylococcal species, however higher levels of repeats were observed in both S. aureus and S. lugdunensis compared to other staphylococcal species. Unexpectedly, sequencing of the internal spacer sequences of individual repeat elements from multiple isolates showed conservation at the sequence level within deep evolutionary lineages of S. aureus. Whilst individual STAR element loci were demonstrated to expand and contract, the sequences associated with each locus were stable and distinct from one another.

Conclusions

The high degree of lineage and locus-specific conservation of these intergenic repeat regions suggests that STAR elements are maintained due to selective or molecular forces with some of these elements having an important role in cell physiology. The high prevalence in two of the more virulent staphylococcal species is indicative of a potential role for STAR elements in pathogenesis.

Characterization of species-specific repeats in 613 prokaryotic species

Prokaryotes are in general believed to possess small, compactly organized genomes, with repetitive sequences forming only a small part of them. Nonetheless, many prokaryotic genomes in fact contain species-specific repeats (>85 bp long genomic sequences with less than 60% identity to other species) as we have previously demonstrated. However, it is not known at present how frequent such species-specific repeats are and what their functional roles in bacterial genomes may be. Therefore, we have conducted a comprehensive survey of prokaryotic species-specific repeats and characterized them to examine as to whether there are functional classes among different repeats or not and how they are mutually related to each other. Of the 613 distinct prokaryotic species analyzed, 97% were found to contain at least one species-specific repeats. It seems interesting to note that the species-specific repeats thus identified appear to be functionally variable in different genomes: in some genomes, they are mostly associated with duplicated protein-coding genes, whereas in some other genomes with rRNA and tRNA genes. Contrary to what may be expected, only one-fourth of the species-specific repeats were found to be associated with mobile genetic elements.

Structuring the bacterial genome: Y1-transposases associated with REP-BIME sequences

REPs are highly repeated intergenic palindromic sequences often clustered into structures called BIMEs including two individual REPs separated by short linker of variable length. They play a variety of key roles in the cell. REPs also resemble the sub-terminal hairpins of the atypical IS200/605 family of insertion sequences which encode Y1 transposases (TnpA(IS200/IS605)). These belong to the HUH endonuclease family, carry a single catalytic tyrosine (Y) and promote single strand transposition. Recently, a new clade of Y1 transposases (TnpA(REP)) was found associated with REP/BIME in structures called REPtrons. It has been suggested that TnpA(REP) is responsible for REP/BIME proliferation over genomes. We analysed and compared REP distribution and REPtron structure in numerous available E. coli and Shigella strains. Phylogenetic analysis clearly indicated that tnpA(REP) was acquired early in the species radiation and was lost later in some strains. To understand REP/BIME behaviour within the host genome, we also studied E. coli K12 TnpA(REP) activity in vitro and demonstrated that it catalyses cleavage and recombination of BIMEs. While TnpA(REP) shared the same general organization and similar catalytic characteristics with TnpA(IS200/IS605) transposases, it exhibited distinct properties potentially important in the creation of BIME variability and in their amplification. TnpA(REP) may therefore be one of the first examples of transposase domestication in prokaryotes.

Wide distribution of O157-antigen biosynthesis gene clusters in Escherichia coli

Most Escherichia coli O157-serogroup strains are classified as enterohemorrhagic E. coli (EHEC), which is known as an important food-borne pathogen for humans. They usually produce Shiga toxin (Stx) 1 and/or Stx2, and express H7-flagella antigen (or nonmotile). However, O157 strains that do not produce Stxs and express H antigens different from H7 are sometimes isolated from clinical and other sources. Multilocus sequence analysis revealed that these 21 O157:non-H7 strains tested in this study belong to multiple evolutionary lineages different from that of EHEC O157:H7 strains, suggesting a wide distribution of the gene set encoding the O157-antigen biosynthesis in multiple lineages. To gain insight into the gene organization and the sequence similarity of the O157-antigen biosynthesis gene clusters, we conducted genomic comparisons of the chromosomal regions (about 59 kb in each strain) covering the O-antigen gene cluster and its flanking regions between six O157:H7/non-H7 strains. Gene organization of the O157-antigen gene cluster was identical among O157:H7/non-H7 strains, but was divided into two distinct types at the nucleotide sequence level. Interestingly, distribution of the two types did not clearly follow the evolutionary lineages of the strains, suggesting that horizontal gene transfer of both types of O157-antigen gene clusters has occurred independently among E. coli strains. Additionally, detailed sequence comparison revealed that some positions of the repetitive extragenic palindromic (REP) sequences in the regions flanking the O-antigen gene clusters were coincident with possible recombination points. From these results, we conclude that the horizontal transfer of the O157-antigen gene clusters induced the emergence of multiple O157 lineages within E. coli and speculate that REP sequences may involve one of the driving forces for exchange and evolution of O-antigen loci.

Identification and characterization of repetitive extragenic palindromes (REP)-associated tyrosine transposases: implications for REP evolution and dynamics in bacterial genomes

Background

Bacterial repetitive extragenic palindromes (REPs) compose a distinct group of genomic repeats. They usually occur in high abundance (>100 copies/genome) and are often arranged in composite repetitive structures - bacterial interspersed mosaic elements (BIMEs). In BIMEs, regularly spaced REPs are present in alternating orientations. BIMEs and REPs have been shown to serve as binding sites for several proteins and suggested to play role in chromosome organization and transcription termination. Their origins are, at present, unknown.

Results

In this report, we describe a novel class of putative transposases related to IS200/IS605 transposase family and we demonstrate that they are obligately associated with bacterial REPs. Open reading frames coding for these REP-associated tyrosine transposases (RAYTs) are always flanked by two REPs in inverted orientation and thus constitute a unit reminiscent of typical transposable elements. Besides conserved residues involved in catalysis of DNA cleavage, RAYTs carry characteristic structural motifs that are absent in typical IS200/IS605 transposases. DNA sequences flanking rayt genes are in one third of examined cases arranged in modular BIMEs. RAYTs and their flanking REPs apparently coevolve with each other. The rayt genes themselves are subject to rapid evolution, substantially exceeding the substitution rate of neighboring genes. Strong correlation was found between the presence of a particular rayt in a genome and the abundance of its cognate REPs.

Conclusions

In light of our findings, we propose that RAYTs are responsible for establishment of REPs and BIMEs in bacterial genomes, as well as for their exceptional dynamics and species-specifity. Conversely, we suggest that BIMEs are in fact a special type of nonautonomous transposable elements, mobilizable by RAYTs.

Modeling DNA-binding of Escherichia coli sigma70 exhibits a characteristic energy landscape around strong promoters

We present a computational model of DNA-binding by σ⁷⁰ in Escherichia coli which allows us to extract the functional characteristics of the wider promoter environment. Our model is based on a measure for the binding energy of σ⁷⁰ to the DNA, which is derived from promoter strength data and used to build up a non-standard weight matrix. Opposed to conventional approaches, we apply the matrix to the environment of 3765 known promoters and consider the average matrix scores to extract the common features. In addition to the expected minimum of the average binding energy at the exact promoter site, we detect two minima shortly upstream and downstream of the promoter. These are likely to occur due to correlation between the two binding sites of σ⁷⁰. Moreover, we observe a characteristic energy landscape in the 500 bp surrounding the transcription start sites, which is more pronounced in groups of strong promoters than in groups of weak promoters. Our subsequent analysis suggests that the characteristic energy landscape is more likely an influence on target search by the RNA polymerase than a result of nucleotide biases in transcription factor binding sites.

Multiple-locus variable-number tandem-repeat analysis for longitudinal survey of sources of Pseudomonas aeruginosa infection in cystic fibrosis patients

In order to identify the source of infection by Pseudomonas aeruginosa in patients with cystic fibrosis (CF), systematic genotyping of isolates is necessary. Multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA) was used to survey the sources of P. aeruginosa infections in a French (Paris, France) pediatric CF center. Between January 2004 and December 2006, 108 patients ages 2 to 21 years who were regularly monitored at the center provided sputum for culture. P. aeruginosa was detected in 46 children, 17 of whom had primary colonization. A total of 163 isolates were recovered. MLVA was improved from a previously published method by the addition of new, informative, and easily typeable markers. Upon genotyping with 15 VNTRs, a total of 39 lineages composed of indistinguishable or closely related isolates, were observed. One of them corresponds to "clone C," which is widely distributed in Europe, and another corresponds to reference strain PA14. Six patients were colonized with two different strains, and the remaining 40 patients were colonized with a single strain. Strains from seven lineages were shared by at least two and up to four patients among a total of 20 patients. The study demonstrates that MLVA is an efficient, easy, and rapid molecular method for epidemiological surveillance for P. aeruginosa infection. The resulting data and strain genetic profiles can be queried on http://bacterial-genotyping.igmors.u-psud.fr.

Bacterial repetitive extragenic palindromic sequences are DNA targets for Insertion Sequence elements

Background

Mobile elements are involved in genomic rearrangements and virulence acquisition, and hence, are important elements in bacterial genome evolution. The insertion of some specific Insertion Sequences had been associated with repetitive extragenic palindromic (REP) elements. Considering that there are a sufficient number of available genomes with described REPs, and exploiting the advantage of the traceability of transposition events in genomes, we decided to exhaustively analyze the relationship between REP sequences and mobile elements.

Results

This global multigenome study highlights the importance of repetitive extragenic palindromic elements as target sequences for transposases. The study is based on the analysis of the DNA regions surrounding the 981 instances of Insertion Sequence elements with respect to the positioning of REP sequences in the 19 available annotated microbial genomes corresponding to species of bacteria with reported REP sequences. This analysis has allowed the detection of the specific insertion into REP sequences for ISPsy8 in Pseudomonas syringae DC3000, ISPa11 in P. aeruginosa PA01, ISPpu9 and ISPpu10 in P. putida KT2440, and ISRm22 and ISRm19 in Sinorhizobium meliloti 1021 genome. Preference for insertion in extragenic spaces with REP sequences has also been detected for ISPsy7 in P. syringae DC3000, ISRm5 in S. meliloti and ISNm1106 in Neisseria meningitidis MC58 and Z2491 genomes. Probably, the association with REP elements that we have detected analyzing genomes is only the tip of the iceberg, and this association could be even more frequent in natural isolates.

Conclusion

Our findings characterize REP elements as hot spots for transposition and reinforce the relationship between REP sequences and genomic plasticity mediated by mobile elements. In addition, this study defines a subset of REP-recognizer transposases with high target selectivity that can be useful in the development of new tools for genome manipulation.

RecN protein and transcription factor DksA combine to promote faithful recombinational repair of DNA double-strand breaks

In rapidly dividing bacterial cells, the machinery for repair of DNA double-strand breaks has to contend not only with the forces driving replication and transmission of the DNA but also its transcription. By exploiting I-SceI homing endonuclease to break the Escherichia coli chromosome at one or more defined locations, we have been able to investigate how these processes are co-ordinated and repair is accomplished. When breaks are induced at a single site, the SOS-inducible RecN protein and the transcription factor DksA combine to promote efficient repair. When induced at two or more, distantly located sites, RecN becomes almost indispensable. Many cells that do survive have extensive deletions of sequences flanking the break, with end points often coinciding with imperfect repeat elements. These findings herald a much greater complexity for chromosome repair than suggested by current mechanistic models and reveal a role for RecN in protecting the chromosome from break-induced chromosome rearrangements.

REP code: defining bacterial identity in extragenic space

Through the analysis of 57 bacterial genomes we have detected repetitive extragenic palindromic DNA sequences (REPs) in 11 species. For a sequence to be considered as REP, the following criteria should be met: (i) It should be extragenic, (ii) palindromic, (iii) of a length between 21 and 65 bases and (iv) should constitute more than 0.5% of the total extragenic space. Species-specific REPs have been found in human pathogens such as Escherichia coli, Salmonella enterica, Neisseria meningitidis, Mycobacterium tuberculosis, Rickettsia conorii and Pseudomonas aeruginosa, the plant pathogen Agrobacterium tumefaciens and the soil bacteria Deinococcus radiodurans, Pseudomonas putida and Sinorhizobium meliloti.

Systematic determination of the mosaic structure of bacterial genomes: species backbone versus strain-specific loops

Background

Public databases now contain multitude of complete bacterial genomes, including several genomes of the same species. The available data offers new opportunities to address questions about bacterial genome evolution, a task that requires reliable fine comparison data of closely related genomes. Recent analyses have shown, using pairwise whole genome alignments, that it is possible to segment bacterial genomes into a common conserved backbone and strain-specific sequences called loops.

Results

Here, we generalize this approach and propose a strategy that allows systematic and non-biased genome segmentation based on multiple genome alignments. Segmentation analyses, as applied to 13 different bacterial species, confirmed the feasibility of our approach to discern the 'mosaic' organization of bacterial genomes. Segmentation results are available through a Web interface permitting functional analysis, extraction and visualization of the backbone/loops structure of documented genomes. To illustrate the potential of this approach, we performed a precise analysis of the mosaic organization of three E. coli strains and functional characterization of the loops.

Conclusion

The segmentation results including the backbone/loops structure of 13 bacterial species genomes are new and available for use by the scientific community at the URL: .

Repetitive extragenic palindromic sequences in the Pseudomonas syringae pv. tomato DC3000 genome: extragenic signals for genome reannotation

Repetitive extragenic palindromic (REPs) sequences were first described in enterobacteriacea and later in Pseudomonas putida. We have detected a new variant (51 base pairs) of REP sequences that appears to be disseminated in more than 300 copies in the Pseudomonas syringae DC3000 genome. The finding of REP sequences in P. syringae confirms the broad presence of this type of repetitive sequence in bacteria. We analyzed the distribution of REP sequences and the structure of the clusters, and we show that palindromy is conserved. REP sequences appear to be allocated to the extragenic space, with a special preference for the intergenic spaces limited by convergent genes, while their presence is scarce between divergent genes. Using REP sequences as markers of extragenicity we re-annotated a set of genes of the P. syringae DC3000 genome demonstrating that REP sequences can be used for refinement of annotation of a genome. The similarity detected between virulence genes from evolutionarily distant pathogenic bacteria suggests the acquisition of clusters of virulence genes by horizontal gene transfer. We did not detect the presence of P. syringae REP elements in the principal pathogenicity gene clusters. This absence suggests that genome fragments lacking REP sequences could point to regions recently acquired from other organisms, and REP sequences might be new tracers for gaining insight into key aspects of bacterial genome evolution, especially when studying pathogenicity acquisition. In addition, as the P. syringae REP sequence is species-specific with respect to the sequenced genomes, it is an exceptional candidate for use as a fingerprint in precise genotyping and epidemiological studies.

Opposite roles of the dmd gene in the control of RNase E and RNase LS activities

When the dmd gene of bacteriophage T4 is defective, expression of middle genes starts normally but drops abruptly. However, the residual expression of middle genes at late stages continues at a higher rate in cells infected with a dmd mutant than with the wild type. In order to understand the complex effects of the dmd gene, we followed changes in the quantity of mRNA from a middle gene, uvsY. The uvsY mRNA was degraded rapidly by RNase LS at middle stages but stabilized at late stages, suggesting that RNase LS targets middle-gene mRNAs only at middle stages. Furthermore, another RNase targeting middle mRNAs at late stages is also suggested to be inactivated when dmd is mutated. We found that RNase E was involved in the degradation of uvsY mRNA. Judging from the processing of gene-32 mRNA, RNase E activity declines after the beginning of the middle stage when dmd is defective.

Subtyping of foodborne and environmental isolates of Escherichia coli by multiplex-PCR, rep-PCR, PFGE, ribotyping and AFLP

A total of 54 isolates were characterized by multiplex-PCR for toxin genes and genotyped using several DNA fingerprinting methods: using repetitive extragenic palindromes (REP) and Box primers (rep-PCR), amplified fragment length polymorphism (AFLP), pulsed-field gel electrophoresis (PFGE) and ribotyping. The known-pathogenic strains tested were from food and clinical samples (34 strains) and included serovars O157:H7, O111:H8, O111:H11, O91:H21 and O55:H7. Two type cultures, Escherichia coli K12 (ATCC 29425) and DUP-101 (ATCC 51739), were included as known non-pathogenic strains and an additional 17 previously unclassified isolates from animal fecal samples. Comparisons of genomic DNA fingerprint patterns using unweighted pair group method with arithmetic averages (UPGMA) cluster analysis of Jaccard similarity indices indicated that all methods tested showed a greater similarity between the E. coli O157:H7 strains than to other isolates. On the basis of these studies, we propose that AFLP, REP-PCR, Box-PCR and ribotyping techniques can all be used for discriminating O157:H7 isolates and are preferred for large-scale screening because of the speed and ease of the methods. The PFGE method is the best to discriminate between subtypes of O157:H7 associated with specific outbreak investigations; however, it is more time consuming and unnecessary if subtyping is not required. There are differences between the dendrograms generated from each method and the relationship between the other strains analyzed. However, the fingerprint profiles of the O157:H7 isolates were virtually identical using REP-PCR and Box-PCR enabling easy distinction of the group. Thus, these typing methods have the potential to aid investigators in identifying the source of an outbreak to prevent or control further spread of E. coli O157:H7.

Palindromic unit-independent transposition of IS1397 in Yersinia pestis

Palindromic units (PUs) are intergenic repeated sequences scattered over the chromosomes of Escherichia coli and several other enterobacteria. In the latter, IS1397, an E. coli insertion sequence specific to PUs, transposes into PUs with sequences close to the E. coli consensus. Reasons for this insertion specificity can relate to either a direct recognition of the target (by its sequence or its structure) by the transposase or an interaction between a specific host protein and the PU target DNA sequence. In this study, we show that for Yersinia pestis, a species deprived of PUs, IS1397 can transpose onto its chromosome, with transpositional hot spots. Our results are in favor of a direct recognition of target DNA by IS1397 transposase.

Identification of a novel family of sequence repeats among prokaryotes

The rapid increase in genomic sequences provides new opportunities for comparative genomics. In this report, we describe a novel family of repeat sequences that is present in Bacteria and Archaea but not in Eukarya. The repeat loci typically consisted of repetitive stretches of nucleotides with a length of 25 to 37 bp alternated by nonrepetitive DNA spacers of approximately equal size as the repeats. The nucleotide sequences and the size of the repeats were highly conserved within a species, but between species the sequences showed no similarity. Due to their characteristic structure, we have designated this family of repeat loci as SPacers Interspersed Direct Repeats (SPIDR). The SPIDR loci were identified in more than forty different prokaryotic species. Individual species such as Mycobacterium tuberculosis contain one SPIDR locus, while other species such as Methanococcus jannaschii contained up to 20 different loci. The number of repeats in a locus varies greatly from two repeats to several dozens of repeats. The SPIDR loci were flanked by a common 300-500-bp leader sequence, which appeared to be conserved within a species but not between species. The SPIDR locus of M. tuberculosis is extensively used for strain typing. The finding of SPIDR loci in other prokaryotes, including the pathogens Salmonella, Campylobacter, and Pasteurella may extend this surveillance to other species.

Genome plasticity in Yersinia pestis

Yersinia pestis, the causative agent of bubonic plague, emerged recently (<20000 years ago) as a clone of Yersinia pseudotuberculosis. There is scant evidence of genome diversity in Y. pestis, although it is possible to differentiate three biovars (antiqua, mediaevalis or orientalis) based on two biochemical tests. There are a few examples of restriction fragment length polymorphisms (RFLPs) within Y. pestis; however, their genetic basis is poorly understood. In this study, six difference regions (DFRs) were identified in Y. pestis, by using subtractive hybridization, which ranged from 4.6 to 19 kb in size. Four of the DFRs are flanked by insertion sequences, and their sequences show similarity to bacterial genes encoding proteins for flagellar synthesis, ABC transport, insect toxicity and bacteriophage functions. The presence or absence of these DFRs (termed the DFR profile) was demonstrated in 78 geographically diverse strains of Y. pestis. Significant genome plasticity was observed among these strains and suggests the acquisition and deletion of these DNA regions during the recent evolution of Y. pestis. Y. pestis biovar orientalis possesses DFR profiles that are different from antiqua and mediaevalis biovars, reflecting the recent origins of this biovar. Whereas some DFR profiles are specific for antiqua and mediaevalis, some DFR profiles are shared by both biovars. Furthermore, the progenitor of Y. pestis, Y. pseudotuberculosis (an enteric pathogen), possesses its own DFR profile. The DFR profiles detailed here demonstrate genome plasticity within Y. pestis, and they imply evolutionary relationships among the three biovars of Y. pestis, as well as between Y. pestis and Y. pseudotuberculosis.

Species-specific repetitive extragenic palindromic (REP) sequences in Pseudomonas putida

Pseudomonas putida KT2440 is a soil bacterium that effectively colonises the roots of many plants and degrades a variety of toxic aromatic compounds. Its genome has recently been sequenced. We describe that a 35 bp sequence with the structure of an imperfect palindrome, originally found repeated three times downstream of the rpoH gene terminator, is detected more than 800 times in the chromosome of this strain. The structure of this DNA segment is analogous to that of the so-called enterobacteriaceae repetitive extragenic palindromic (REP) sequences, although its sequence is different. Computer-assisted analysis of the presence and distribution of this repeated sequence in the P.putida chromosome revealed that in at least 80% of the cases the sequence is extragenic, and in 82% of the cases the distance of this extragenic element to the end of one of the neighbouring genes was <100 bp. This 35 bp element can be found either as a single element, as pairs of elements, or sometimes forming clusters of up to five elements in which they alternate orientation. PCR scanning of chromosomes from different isolates of Pseudomonas sp. strains using oligonucleotides complementary to the most conserved region of this sequence shows that it is only present in isolates of the species P.putida. For this reason we suggest that the P.putida 35 bp element is a distinctive REP sequence in P.putida. This is the first time that REP sequences have been described and characterised in a group of non-enterobacteriaceae.

Identification of genes that are associated with DNA repeats in prokaryotes

Using in silico analysis we studied a novel family of repetitive DNA sequences that is present among both domains of the prokaryotes (Archaea and Bacteria), but absent from eukaryotes or viruses. This family is characterized by direct repeats, varying in size from 21 to 37 bp, interspaced by similarly sized non-repetitive sequences. To appreciate their characteri-stic structure, we will refer to this family as the clustered regularly interspaced short palindromic repeats (CRISPR). In most species with two or more CRISPR loci, these loci were flanked on one side by a common leader sequence of 300-500 b. The direct repeats and the leader sequences were conserved within a species, but dissimilar between species. The presence of multiple chromosomal CRISPR loci suggests that CRISPRs are mobile elements. Four CRISPR-associated (cas) genes were identified in CRISPR-containing prokaryotes that were absent from CRISPR-negative prokaryotes. The cas genes were invariably located adjacent to a CRISPR locus, indicating that the cas genes and CRISPR loci have a functional relationship. The cas3 gene showed motifs characteristic for helicases of the superfamily 2, and the cas4 gene showed motifs of the RecB family of exonucleases, suggesting that these genes are involved in DNA metabolism or gene expression. The spatial coherence of CRISPR and cas genes may stimulate new research on the genesis and biological role of these repeats and genes.

Computational identification of noncoding RNAs in E. coli by comparative genomics

Some genes produce noncoding transcripts that function directly as structural, regulatory, or even catalytic RNAs [1, 2]. Unlike protein-coding genes, which can be detected as open reading frames with distinctive statistical biases, noncoding RNA (ncRNA) gene sequences have no obvious inherent statistical biases [3]. Thus, genome sequence analyses reveal novel protein-coding genes, but any novel ncRNA genes remain invisible. Here, we describe a computational comparative genomic screen for ncRNA genes. The key idea is to distinguish conserved RNA secondary structures from a background of other conserved sequences using probabilistic models of expected mutational patterns in pairwise sequence alignments. We report the first whole-genome screen for ncRNA genes done with this method, in which we applied it to the "intergenic" spacers of Escherichia coli using comparative sequence data from four related bacteria. Starting from >23,000 conserved interspecies pairwise alignments, the screen predicted 275 candidate structural RNA loci. A sample of 49 candidate loci was assayed experimentally. At least 11 loci expressed small, apparently noncoding RNA transcripts of unknown function. Our computational approach may be used to discover structural ncRNA genes in any genome for which appropriate comparative genome sequence data are available.

Transposition of IS1397 in the family Enterobacteriaceae and first characterization of ISKpn1, a new insertion sequence associated with Klebsiella pneumoniae palindromic units

IS1397 and ISKpn1 are IS3 family members which are specifically inserted into the loop of palindromic units (PUs). IS1397 is shown to transpose into PUs with sequences close or identical to the Escherichia coli consensus, even in other enterobacteria (Salmonella enterica serovar Typhimurium, Klebsiella pneumoniae, and Klebsiella oxytoca). Moreover, we show that homologous intergenic regions containing PUs constitute IS1397 transpositional hot spots, despite bacterial interspersed mosaic element structures that differ among the three species. ISKpn1, described here for the first time, is specific for PUs from K. pneumoniae, in which we discovered it. A sequence comparison between the two insertion sequences allowed us to define a motif possibly accounting for their specificity.

The colanic acid gene cluster of Salmonella enterica has a complex history

The colanic acid gene cluster of Salmonella enterica LT2 was sequenced and compared with that of Escherichia coli K-12. The two clusters are similar with divergence slightly higher than average for genes of the two species. The cluster was divided into four blocks by GC content and seems likely to have transferred from a higher GC content species to the ancestor of E. coli and S. enterica. All 19 genes of K-12 and 13 genes of LT2 appear to have undergone random genetic drift with amelioration of the GC content. However, in the case of S. enterica, we believe that the six genes of the GDP-fucose pathway group were replaced relatively recently by genes closely related to those of the original donor species. Two repetitive elements were observed: a bacterial interspersed mosaic element in the intergenic region between wzx and wcaK in K-12 only and a RSA (repetitive sequence element) sequence between wcaJ and wzx in LT2 only.

IS1630 of Mycoplasma fermentans, a novel IS30-type insertion element that targets and duplicates inverted repeats of variable length and sequence during insertion

A new insertion sequence (IS) of Mycoplasma fermentans is described. This element, designated IS1630, is 1,377 bp long and has 27-bp inverted repeats at the termini. A single open reading frame (ORF), predicted to encode a basic protein of either 366 or 387 amino acids (depending on the start codon utilized), occupies most of this compact element. The predicted translation product of this ORF has homology to transposases of the IS30 family of IS elements and is most closely related (27% identical amino acid residues) to the product of the prototype of the group, IS30. Multiple copies of IS1630 are present in the genomes of at least two M. fermentans strains. Characterization and comparison of nine copies of the element revealed that IS1630 exhibits unusual target site specificity and, upon insertion, duplicates target sequences in a manner unlike that of any other IS element. IS1630 was shown to have the striking ability to target and duplicate inverted repeats of variable length and sequence during transposition. IS30-type elements typically generate 2- or 3-bp target site duplications, whereas those created by IS1630 vary between 19 and 26 bp. With the exception of two recently reported IS4-type elements which have the ability to generate variable large duplications (B. B. Plikaytis, J. T. Crawford, and T. M. Shinnick, J. Bacteriol. 180:1037-1043, 1998; E. M. Vilei, J. Nicolet, and J. Frey, J. Bacteriol. 181:1319-1323, 1999), such large direct repeats had not been observed for other IS elements. Interestingly, the IS1630-generated duplications are all symmetrical inverted repeat sequences that are apparently derived from rho-independent transcription terminators of neighboring genes. Although the consensus target site for IS30 is almost palindromic, individual target sites possess considerably less inverted symmetry. In contrast, IS1630 appears to exhibit an increased stringency for inverted repeat recognition, since the majority of target sites had no mismatches in the inverted repeat sequences. In the course of this study, an additional copy of the previously identified insertion sequence ISMi1 was cloned. Analysis of the sequence of this element revealed that the transposase encoded by this element is more than 200 amino acid residues longer and is more closely related to the products of other IS3 family members than had previously been recognized. A potential site for programmed translational frameshifting in ISMi1 was also identified.

IS1397 is active for transposition into the chromosome of Escherichia coli K-12 and inserts specifically into palindromic units of bacterial interspersed mosaic elements

We demonstrate that IS1397, a putative mobile genetic element discovered in natural isolates of Escherichia coli, is active for transposition into the chromosome of E. coli K-12 and inserts specifically into palindromic units, also called repetitive extragenic palindromes, the basic element of bacterial interspersed mosaic elements (BIMEs), which are found in intergenic regions of enterobacteria closely related to E. coli and Salmonella. We could not detect transposition onto a plasmid carrying BIMEs. This unprecedented specificity of insertion into a well-characterized chromosomal intergenic repeated element and its evolutionary implications are discussed.

Complete sequence of plasmid pLH1 from lactobacillus helveticus ATCC15009: analysis reveals the presence of regions homologous to other native plasmids from the host strain

The complete sequence for plasmid pLH1 from Lactobacillus helveticus ATCC15009 has been determined. Analysis of the 19,360-bp primary sequence revealed a putative replication origin and initiation protein, information that could provide the basis for the construction of cloning vectors for L. helveticus. Evidence that pLH1 is theta-replicating could be deduced from the plasmid size, from the homology to the replication protein of the Bacillus natto theta-replicating plasmid pLS32, and from the identification of a putative resolvase gene (orf-195). Although 14 open reading frames capable of encoding polypeptides longer than 100 amino acids were identified, none, on the basis of homology with known sequences, appeared to encode a well-characterized trait relevant to milk fermentation. Plasmid pLH1 revealed regions of identity with the smaller cryptic plasmids (pLH2 and pLH3) from the same strain and with other tracts of DNA, including insertion sequence elements, from a variety of other lactic acid bacteria. The presence of such regions provides a basis for developing an explanation of the phenotypic variability observed in these bacteria. The plasmid also appears to possess a number of genetic elements present in other lactic acid bacterial plasmids, conservation of which would be consistent with an important functional or evolutionary role. It could be argued that the plasmid complement of L. helveticus ATCC15009 consists of parasitic entities concerned only with their own replication and survival.

IS elements as constituents of bacterial genomes

We provide here an overview of our present understanding of the distribution of different insertion sequences (ISs) within bacterial genomes (both chromosomes and plasmids). This is at present fragmentary and a significant effort is needed in the analysis of the increasing number of genomes whose sequence has been determined. We also consider some of the properties of ISs which are important in their role of assembling, reassorting, and transmitting groups of genes.

Short repeats and IS elements in the extremely radiation-resistant bacterium Deinococcus radiodurans and comparison to other bacterial species

Computer analysis of the complete genome of Deinococcus radiodurans R1 has shown that the number of insertion sequences (ISs) and small noncoding repeats (SNRs) it contains is very high, and comparable with those of Escherichia coli. IS elements and several families of SNRs are described, together with their possible function in the D. radiodurans genome.

Short palindromic repetitive DNA elements in enterobacteria: a survey

We present a survey of short palindromic repetitive elements in enterobacteria. Seven families are presented. Five were already known (RSA, IRU, 29-bp repeats, BIMEs and boxC), and their properties are updated; in particular, a new composite element is shown to include the formerly identified boxC repeats. Two repetitions, YPAL1 and YPAL2, found primarily in Yersinia, are described here for the first time.

Functional and evolutionary roles of long repeats in prokaryotes

Most recently published complete bacterial genomes have revealed unexpectedly high numbers of long strict repeats. In this article we discuss the various functional and evolutionary roles of these repeats, focusing in particular on their role in terms of genome stability, gene transfer, and antigenic variation.

Repeated extragenic sequences in prokaryotic genomes: a proposal for the origin and dynamics of the RUP element in Streptococcus pneumoniae

A survey of all Streptococcus pneumoniae GenBank/EMBL DNA sequence entries and of the public domain sequence (representing more than 90% of the genome) of an S. pneumoniae type 4 strain allowed identification of 108 copies of a 107-bp-long highly repeated intergenic element called RUP (for repeat unit of pneumococcus). Several features of the element, revealed in this study, led to the proposal that RUP is an insertion sequence (IS)-derivative that could still be mobile. Among these features are: (1) a highly significant homology between the terminal inverted repeats (IRs) of RUPs and of IS630-Spn1, a new putative IS of S. pneumoniae; and (2) insertion at a TA dinucleotide, a characteristic target of several members of the IS630 family. Trans-mobilization of RUP is therefore proposed to be mediated by the transposase of IS630-Spn1. To account for the observation that RUPs are distributed among four subtypes which exhibit different degrees of sequence homogeneity, a scenario is invoked based on successive stages of RUP mobility and non-mobility, depending on whether an active transposase is present or absent. In the latter situation, an active transposase could be reintroduced into the species through natural transformation. Examination of sequences flanking RUP revealed a preferential association with ISs. It also provided evidence that RUPs promote sequence rearrangements, thereby contributing to genome flexibility. The possibility that RUP preferentially targets transforming DNA of foreign origin and subsequently favours disruption/rearrangement of exogenous sequences is discussed.

Transposable elements as a molecular evolutionary force

This essay addresses the paradoxes of the complex and highly redundant genomes. The central theses developed are that: (1) the distinctive feature of complex genomes is the existence of epigenetic mechanisms that permit extremely high levels of both tandem and dispersed redundancy; (2) the special contribution of transposable elements is to modularize the genome; and (3) the labilizing forces of recombination and transposition are just barely contained, giving a dynamic genetic system of ever increasing complexity that verges on the chaotic.

Insertion sequences

Insertion sequences (ISs) constitute an important component of most bacterial genomes. Over 500 individual ISs have been described in the literature to date, and many more are being discovered in the ongoing prokaryotic and eukaryotic genome-sequencing projects. The last 10 years have also seen some striking advances in our understanding of the transposition process itself. Not least of these has been the development of various in vitro transposition systems for both prokaryotic and eukaryotic elements and, for several of these, a detailed understanding of the transposition process at the chemical level. This review presents a general overview of the organization and function of insertion sequences of eubacterial, archaebacterial, and eukaryotic origins with particular emphasis on bacterial elements and on different aspects of the transposition mechanism. It also attempts to provide a framework for classification of these elements by assigning them to various families or groups. A total of 443 members of the collection have been grouped in 17 families based on combinations of the following criteria: (i) similarities in genetic organization (arrangement of open reading frames); (ii) marked identities or similarities in the enzymes which mediate the transposition reactions, the recombinases/transposases (Tpases); (iii) similar features of their ends (terminal IRs); and (iv) fate of the nucleotide sequence of their target sites (generation of a direct target duplication of determined length). A brief description of the mechanism(s) involved in the mobility of individual ISs in each family and of the structure-function relationships of the individual Tpases is included where available.

Parallel and divergent genotypic evolution in experimental populations of Ralstonia sp

Genetic rearrangements within a population of bacteria were analyzed to understand the degree of divergence occurring after experimental evolution. We used 18 replicate populations founded from Ralstonia sp. strain TFD41 that had been propagated for 1,000 generations with 2,4-dichlorophenoxyacetic acid (2,4-D) as the carbon source. Genetic divergence was examined by restriction fragment length polymorphism analysis of the incumbent plasmid that carries the 2,4-D catabolic genes and by amplification of random regions of the genome via PCR. In 18 evolved clones examined, we observed duplication within the plasmid, including the tfdA gene, which encodes a 2,4-D dioxygenase that catalyzes the first step in the 2,4-D catabolic pathway. In 71 of 72 evolved clones, a common 2.4-kb PCR product was lost when genomic fingerprints produced by PCR amplification using degenerate primers based on repetitive extragenic palindromic (REP) sequences (REP-PCR) were compared. The nucleotide sequence of the 2.4-kb PCR product has homology to the TRAP (tripartite ATP-independent periplasmic) solute transporter gene family. Hybridization of the 2. 4-kb REP-PCR product from the ancestor to genomic DNA from the evolved populations showed that the loss of the PCR product resulted from deletions in the genome. Deletions in the plasmid and presence and/or absence of other REP-PCR products were also found in these clones but at much lower frequencies. The common and uncommon genetic changes observed show that both parallel and divergent genotypic evolution occurred in replicate populations of this bacterium.