Site-specific recombination by the DDE family member mobile element IS30 transposase

The dynamic network of IS30 transposition pathways

The E. coli element IS30 has adopted the copy-out-paste-in transposition mechanism that is prevalent in a number of IS-families. As an initial step, IS30 forms free circular transposition intermediates like IS minicircles or tandem IS-dimers by joining the inverted repeats of a single element or two, sometimes distantly positioned IS copies, respectively. Then, the active IR-IR junction of these intermediates reacts with the target DNA, which generates insertions, deletions, inversions or cointegrates. The element shows dual target specificity as it can insert into hot spot sequences or next to its inverted repeats. In this study the pathways of rearrangements of transposition-derived cointegrate-like structures were examined. The results showed that the probability of further rearrangements in these structures depends on whether the IS elements are flanked by hot spot sequences or take part in an IR-IR junction. The variability of the deriving products increases with the number of simultaneously available IRs and IR-IR joints in the cointegrates or the chromosome. Under certain conditions, the parental structures whose transposition formed the cointegrates are restored and persist among the rearranged products. Based on these findings, a novel dynamic model has been proposed for IS30, which possibly fits to other elements that have adopted the same transposition mechanism. The model integrates the known transposition pathways and the downstream rearrangements occurring after the formation of different cointegrate-like structures into a complex network. Important feature of this network is the presence of “feedback loops” and reversible transposition rearrangements that can explain how IS30 generates variability and preserves the original genetic constitution in the bacterial population, which contributes to the adaptability and evolution of host bacteria.

Phages infecting Faecalibacterium prausnitzii belong to novel viral genera that help to decipher intestinal viromes

BACKGROUND

Viral metagenomic studies have suggested a role for bacteriophages in intestinal dysbiosis associated with several human diseases. However, interpretation of viral metagenomic studies is limited by the lack of knowledge of phages infecting major human gut commensal bacteria, such as Faecalibacterium prausnitzii, a bacterial symbiont repeatedly found depleted in inflammatory bowel disease (IBD) patients. In particular, no complete genomes of phages infecting F. prausnitzii are present in viral databases.

METHODS

We identified 18 prophages in 15 genomes of F. prausnitzii, used comparative genomics to define eight phage clades, and annotated the genome of the type phage of each clade. For two of the phages, we studied prophage induction in vitro and in vivo in mice. Finally, we aligned reads from already published viral metagenomic data onto the newly identified phages.

RESULTS

We show that each phage clade represents a novel viral genus and that a surprisingly large fraction of them (10 of the 18 phages) codes for a diversity-generating retroelement, which could contribute to their adaptation to the digestive tract environment. We obtained either experimental or in silico evidence of activity for at least one member of each genus. In addition, four of these phages are either significantly more prevalent or more abundant in stools of IBD patients than in those of healthy controls.

CONCLUSION

Since IBD patients generally have less F. prausnitzii in their microbiota than healthy controls, the higher prevalence or abundance of some of its phages may indicate that they are activated during disease. This in turn suggests that phages could trigger or aggravate F. prausnitzii depletion in patients. Our results show that prophage detection in sequenced strains of the microbiota can usefully complement viral metagenomic studies.

Complete genetic analysis of plasmids carrying mcr-1 and other resistance genes in an Escherichia coli isolate of animal origin

Objectives

To investigate the genetic features of three plasmids recovered from an MCR-1 and ESBL-producing Escherichia coli strain, HYEC7, and characterize the transmission mechanism of mcr-1 .

Methods

The genetic profiles of three plasmids were determined by PCR, S1-PFGE, Southern hybridization and WGS analysis. The ability of the mcr-1 -bearing plasmid to undergo conjugation was also assessed. The mcr-1 -bearing transposon Tn 6330 was characterized by PCR and DNA sequencing.

Results

Complete sequences of three plasmids were obtained. A non-conjugative phage P7-like plasmid, pHYEC7- mcr1 , was found to harbour the mcr-1 -bearing transposon Tn 6330 , which could be excised from the plasmid by generating a circular intermediate harbouring mcr-1 and the IS Apl1 element. The insertion of the circular intermediate into another plasmid, pHYEC7-IncHI2, could form pHNSHP45-2, the original IncHI2-type mcr-1 -carrying plasmid that was reported. The third plasmid, pHYEC7-110, harboured two replicons, IncX1 and IncFIB, and comprised multiple antimicrobial resistance mobile elements, some of which were shared by pHYEC7-IncHI2.

Conclusions

The Tn 6330 element located in the phage-like plasmid pHYEC7- mcr1 could be excised from the plasmid and formed a circular intermediate that could be integrated into plasmids containing the IS Apl1 element. This phenomenon indicated that Tn 6330 is a key element responsible for widespread dissemination of mcr-1 among various types of plasmids and bacterial chromosomes. The dissemination rate of such an element may be further enhanced upon translocation into phage-like vectors, which may also be transmitted via transduction events.

Everyman's Guide to Bacterial Insertion Sequences

The number and diversity of known prokaryotic insertion sequences (IS) have increased enormously since their discovery in the late 1960s. At present the sequences of more than 4000 different IS have been deposited in the specialized ISfinder database. Over time it has become increasingly apparent that they are important actors in the evolution of their host genomes and are involved in sequestering, transmitting, mutating and activating genes, and in the rearrangement of both plasmids and chromosomes. This review presents an overview of our current understanding of these transposable elements (TE), their organization and their transposition mechanism as well as their distribution and genomic impact. In spite of their diversity, they share only a very limited number of transposition mechanisms which we outline here. Prokaryotic IS are but one example of a variety of diverse TE which are being revealed due to the advent of extensive genome sequencing projects. A major conclusion from sequence comparisons of various TE is that frontiers between the different types are becoming less clear. We detail these receding frontiers between different IS-related TE. Several, more specialized chapters in this volume include additional detailed information concerning a number of these.

In a second section of the review, we provide a detailed description of the expanding variety of IS, which we have divided into families for convenience. Our perception of these families continues to evolve and families emerge regularly as more IS are identified. This section is designed as an aid and a source of information for consultation by interested specialist readers.

Site-directed (IS30-FljA) transposon mutagenesis system to produce nonflagellated mutants of Salmonella Enteritidis

Site-directed integration/mutagenesis systems are used to carry out targeted transpositions on DNA. The well-characterized IS30-element and its transposase have numerous advantages that predestine it to be a good candidate for such applications. In order to generate nonflagellated mutants of Salmonella Enteritidis, a new site-directed mutagenesis system has been developed and applied. The system was constructed based on the assumption that the DNA-binding FljA component of the fusion transposase would bind to its target (the operator of fliC), and as a consequence, insertions could be concentrated in the flagellin operon. The system consists of two components: one expresses the fusion transposase and the other is an integration donor plasmid harbouring the (IS30)(2) reactive structure. The application of this site-directed mutagenesis system on a strain of S. Enteritidis 11 (SE11) resulted in several nonmotile mutants with fliD insertion that could serve as negatively markered vaccine candidates. Analysis of less motile mutants generated by the fusion transposase revealed further hot spot sequences preferred by the fusion construct.

Functional organization of the inverted repeats of IS30

The mobile element IS30 has 26-bp imperfect terminal inverted repeats (IRs) that are indispensable for transposition. We have analyzed the effects of IR mutations on both major transposition steps, the circle formation and integration of the abutted ends, characteristic for IS30. Several mutants show strikingly different phenotypes if the mutations are present at one or both ends and differentially influence the transposition steps. The two IRs are equivalent in the recombination reactions and contain several functional regions. We have determined that positions 20 to 26 are responsible for binding of the N-terminal domain of the transposase and the formation of a correct 2-bp spacer between the abutted ends. However, integration is efficient without this region, suggesting that a second binding site for the transposase may exist, possibly within the region from 4 to 11 bp. Several mutations at this part of the IRs, which are highly conserved in the IS30 family, considerably affected both major transposition steps. In addition, positions 16 and 17 seem to be responsible for distinguishing the IRs of related insertion sequences by providing specificity for the transposase to recognize its cognate ends. Finally, we show both in vivo and in vitro that position 3 has a determining role in the donor function of the ends, especially in DNA cleavage adjacent to the IRs. Taken together, the present work provides evidence for a more complex organization of the IS30 IRs than was previously suggested.

Insertion sequence content reflects genome plasticity in strains of the root nodule actinobacterium Frankia

Background

Genome analysis of three Frankia sp. strains has revealed a high number of transposable elements in two of the strains. Twelve out of the 20 major families of bacterial Insertion Sequence (IS) elements are represented in the 148 annotated transposases of Frankia strain HFPCcI3 (CcI3) comprising 3% of its total coding sequences (CDS). EAN1pec (EAN) has 183 transposase ORFs from 13 IS families comprising 2.2% of its CDS. Strain ACN14a (ACN) differs significantly from the other strains with only 33 transposase ORFs (0.5% of the total CDS) from 9 IS families.

Results

Insertion sequences in the Frankia genomes were analyzed using BLAST searches, PHYML phylogenies and the IRF (Inverted Repeat Finder) algorithms. To identify putative or decaying IS elements, a PSI-TBLASTN search was performed on all three genomes, identifying 36%, 39% and 12% additional putative transposase ORFs than originally annotated in strains CcI3, EAN and ACN, respectively. The distribution of transposase ORFs in each strain was then analysed using a sliding window, revealing significant clustering of elements in regions of the EAN and CcI3 genomes. Lastly the three genomes were aligned with the MAUVE multiple genome alignment tool, revealing several Large Chromosome Rearrangement (LCR) events; many of which correlate to transposase clusters.

Conclusion

Analysis of transposase ORFs in Frankia sp. revealed low inter-strain diversity of transposases, suggesting that the majority of transposase proliferation occurred without recent horizontal transfer of novel mobile elements from outside the genus. Exceptions to this include representatives from the IS3 family in strain EAN and seven IS4 transposases in all three strains that have a lower G+C content, suggesting recent horizontal transfer. The clustering of transposase ORFs near LCRs revealed a tendency for IS elements to be associated with regions of chromosome instability in the three strains. The results of this study suggest that IS elements may help drive chromosome differences in different Frankia sp. strains as they have adapted to a variety of hosts and environments.

Integrative and sequence characteristics of a novel genetic element, ICE6013, in Staphylococcus aureus

A survey of chromosomal variation in the ST239 clonal group of methicillin-resistant Staphylococcus aureus (MRSA) revealed a novel genetic element, ICE6013. The element is 13,354 bp in length, excluding a 6,551-bp Tn552 insertion. ICE6013 is flanked by 3-bp direct repeats and is demarcated by 8-bp imperfect inverted repeats. The element was present in 6 of 15 genome-sequenced S. aureus strains, and it was detected using genetic markers in 19 of 44 diverse MRSA and methicillin-susceptible strains and in all 111 ST239 strains tested. Low integration site specificity was discerned. Multiple chromosomal copies and the presence of extrachromosomal circular forms of ICE6013 were detected in various strains. The circular forms included 3-bp coupling sequences, located between the 8-bp ends of the element, that corresponded to the 3-bp direct repeats flanking the chromosomal forms. ICE6013 is predicted to encode 15 open reading frames, including an IS30-like DDE transposase in place of a Tyr/Ser recombinase and homologs of gram-positive bacterial conjugation components. Further sequence analyses indicated that ICE6013 is more closely related to ICEBs1 from Bacillus subtilis than to the only other potential integrative conjugative element known from S. aureus, Tn5801. Evidence of recombination between ICE6013 elements is also presented. In summary, ICE6013 is the first member of a new family of active, integrative genetic elements that are widely dispersed within S. aureus strains.

Characterization of the variants, flanking genes, and promoter activity of the Leifsonia xyli subsp. cynodontis insertion sequence IS1237

We performed a comprehensive study of the distribution and function of an insertion sequence (IS) element, IS1237, in the genome of Leifsonia xyli subsp. cynodontis, a useful genetic carrier for expressing beneficial foreign genes in plants. Two shorter IS1237 isoforms, IS1237d1 and IS1237d2 resulting from precise deletion between two nonperfect repeats, were found in the bacterial genome at a level that was one-fifth the level of wild-type IS1237. Both the genome and native plasmid pCXC100 harbor a truncated toxin-antitoxin cassette that is precisely fused with a 5'-truncated IS1237 sequence at one nonperfect repeat, indicating that it is a hot site for DNA rearrangement. Nevertheless, no transposition activity was detected when the putative transposase of IS1237 was overexpressed in Escherichia coli. Using thermal asymmetric interlaced PCR, we identified 13 upstream and 10 downstream unique flanking sequences, and two pairs of these sequences were from the same loci, suggesting that IS1237 has up to 65 unique loci in the L. xyli subsp. cynodontis chromosome. The presence of TAA or TTA direct repeat sequences at most insertion sites indicated that IS1237 inserts into the loci by active transposition. IS1237 showed a high propensity for insertion into other IS elements, such as ISLxc1 and ISLxc2, which could offer IS1237 a nonautonomous transposition pathway through the host IS elements. Interestingly, we showed that IS1237 has a strong promoter at the 3' end and a weak promoter at the 5' end, and both promoters promote the transcription of adjacent genes in different gram-positive bacteria. The high-copy-number nature of IS1237 and its promoter activity may contribute to bacterial fitness.

Transposition and target specificity of the typical IS30 family element IS1655 from Neisseria meningitidis

We have analysed the transposition and target selection strategy of IS1655, a typical IS30 family member resident in Neisseria meningitidis. We have redefined IS1655 as a 1080 bp long element with 25 bp imperfect inverted repeats (IRs), which generates a 3 bp target duplication and have shown that it transposes using an intermediate with abutted IRs separated by 2 bp. IS1655 exhibits bipartite target specificity inserting preferentially either next to sequences similar to its IRs or into an unrelated but well defined sequence. IR-targeting leads to the formation of a new junction in which the targeted IR and one of the donor IRs are separated by 2 bp. The non-IR targets were characterized as an imperfect 19 bp palindrome in which the central five positions show slight GC excess and the distal region is AT-rich. Artificial targets designed according to the consensus were recognized by the element as hot spots for insertion. The organization of IS1655 is similar to that of other IS30 family members. Moreover, it shows striking similarity to IS30 in transposition strategy even though their transposases differ in their N-terminal regions, which, for IS30, appears to determine target specificity. Comparative analysis of the transposases and the evolutionary aspects of sequence variants are also briefly discussed.

Piv site-specific invertase requires a DEDD motif analogous to the catalytic center of the RuvC Holliday junction resolvases

Piv, a unique prokaryotic site-specific DNA invertase, is related to transposases of the insertion elements from the IS110/IS492 family and shows no similarity to the site-specific recombinases of the tyrosine- or serine-recombinase families. Piv tertiary structure is predicted to include the RNase H-like fold that typically encompasses the catalytic site of the recombinases or nucleases of the retroviral integrase superfamily, including transposases and RuvC-like Holliday junction resolvases. Analogous to the DDE and DEDD catalytic motifs of transposases and RuvC, respectively, four Piv acidic residues D9, E59, D101, and D104 appear to be positioned appropriately within the RNase H fold to coordinate two divalent metal cations. This suggests mechanistic similarity between site-specific inversion mediated by Piv and transposition or endonucleolytic reactions catalyzed by enzymes of the retroviral integrase superfamily. The role of the DEDD motif in Piv catalytic activity was addressed using Piv variants that are substituted individually or multiply at these acidic residues and assaying for in vivo inversion, intermolecular recombination, and DNA binding activities. The results indicate that all four residues of the DEDD motif are required for Piv catalytic activity. The DEDD residues are not essential for inv recombination site recognition and binding, but this acidic tetrad does appear to contribute to the stability of Piv-inv interactions. On the basis of these results, a working model for Piv-mediated inversion that includes resolution of a Holliday junction is presented.

Analysis of the N-terminal DNA binding domain of the IS30 transposase

IS30 is the founding member of a large family of widely spread bacterial insertion sequences with closely related transposases. The N-terminal end of the IS30 transposase had been shown to retain sequence-specific DNA binding activity and to protect the IS30 terminal inverted repeats. Structural predictions revealed the presence of a helix-helix-turn-helix motif (H-HTH2) which, in the case of IS30, is preceded by an additional helix-turn-helix motif (HTH1). Analysis of deletion and point mutants in this region revealed that both motifs are important for IS30 transposition. IS30 exhibits two types of insertion specificity preferring either a 24 bp palindromic hot-spot (GOHS) or sequences resembling its ends [left and right terminal inverted repeat (IRL and IRR)]. Results are presented suggesting that the HTH1 region is required for GOHS targeting and interferes with the inverted repeat (IR) targeting. On the other hand, H-HTH2 appears to be required for both. The binding activities of the mutant proteins to the terminal IS30 IRs as measured by gel retardation correlated well with these results. Furthermore, close inspection of the H-HTH2 region revealed significant amino acid identity with a similar predicted secondary structure carried by the transcriptional regulator FixJ of Sinorhizobium meliloti and involved in FixJ binding to its target sequence. This suggests that FixJ and IS30 transposase share similar sequence-specific DNA binding mechanisms.

Evolutionary dynamics of insertion sequences in Helicobacter pylori

Prokaryotic insertion sequence (IS) elements behave like parasites in terms of their ability to invade and proliferate in microbial gene pools and like symbionts when they coevolve with their bacterial hosts. Here we investigated the evolutionary history of IS605 and IS607 of Helicobacter pylori, a genetically diverse gastric pathogen. These elements contain unrelated transposase genes (orfA) and also a homolog of the Salmonella virulence gene gipA (orfB). A total of 488 East Asian, Indian, Peruvian, and Spanish isolates were screened, and 18 and 14% of them harbored IS605 and IS607, respectively. IS605 nucleotide sequence analysis (n = 42) revealed geographic subdivisions similar to those of H. pylori; the geographic subdivision was blurred, however, due in part to homologous recombination, as indicated by split decomposition and homoplasy tests (homoplasy ratio, 0.56). In contrast, the IS607 populations (n = 44) showed strong geographic subdivisions with less homologous recombination (homoplasy ratio, 0.2). Diversifying selection (ratio of nonsynonymous change to synonymous change, >>1) was evident in approximately 15% of the IS605 orfA codons analyzed but not in the IS607 orfA codons. Diversifying selection was also evident in approximately 2% of the IS605 orfB and approximately 10% of the IS607 orfB codons analyzed. We suggest that the evolution of these elements reflects selection for optimal transposition activity in the case of IS605 orfA and for interactions between the OrfB proteins and other cellular constituents that potentially contribute to bacterial fitness. Taken together, similarities in IS elements and H. pylori population genetic structures and evidence of adaptive evolution in IS elements suggest that there is coevolution between these elements and their bacterial hosts.