Target joining of duplicated insertion sequence IS21 is assisted by IstB protein in vitro

Molecular basis for transposase activation by a dedicated AAA+ ATPase

Transposases drive chromosomal rearrangements and the dissemination of drug-resistance genes and toxins1-3. Although some transposases act alone, many rely on dedicated AAA+ ATPase subunits that regulate site selectivity and catalytic function through poorly understood mechanisms. Using IS21 as a model transposase system, we show how an ATPase regulator uses nucleotide-controlled assembly and DNA deformation to enable structure-based site selectivity, transposase recruitment, and activation and integration. Solution and cryogenic electron microscopy studies show that the IstB ATPase self-assembles into an autoinhibited pentamer of dimers that tightly curves target DNA into a half-coil. Two of these decamers dimerize, which stabilizes the target nucleic acid into a kinked S-shaped configuration that engages the IstA transposase at the interface between the two IstB oligomers to form an approximately 1 MDa transpososome complex. Specific interactions stimulate regulator ATPase activity and trigger a large conformational change on the transposase that positions the catalytic site to perform DNA strand transfer. These studies help explain how AAA+ ATPase regulators-which are used by classical transposition systems such as Tn7, Mu and CRISPR-associated elements-can remodel their substrate DNA and cognate transposases to promote function.

IS21 family transposase cleaved donor complex traps two right-handed superhelical crossings

Transposases are ubiquitous enzymes that catalyze DNA rearrangement events with broad impacts on gene expression, genome evolution, and the spread of drug-resistance in bacteria. Here, we use biochemical and structural approaches to define the molecular determinants by which IstA, a transposase present in the widespread IS21 family of mobile elements, catalyzes efficient DNA transposition. Solution studies show that IstA engages the transposon terminal sequences to form a high-molecular weight complex and promote DNA integration. A 3.4 Å resolution structure of the transposase bound to transposon ends corroborates our biochemical findings and reveals that IstA self-assembles into a highly intertwined tetramer that synapses two supercoiled terminal inverted repeats. The three-dimensional organization of the IstA•DNA cleaved donor complex reveals remarkable similarities with retroviral integrases and classic transposase systems, such as Tn7 and bacteriophage Mu, and provides insights into IS21 transposition.

Unstable Mechanisms of Resistance to Inhibitors of Escherichia coli Lipoprotein Signal Peptidase

Despite increasing evidence suggesting that antibiotic heteroresistance can lead to treatment failure, the significance of this phenomena in the clinic is not well understood, because many clinical antibiotic susceptibility testing approaches lack the resolution needed to reliably classify heteroresistant strains. Here we present G0790, a new globomycin analog and potent inhibitor of the Escherichia coli type II signal peptidase LspA. We demonstrate that in addition to previously known mechanisms of resistance to LspA inhibitors, unstable genomic amplifications containing lspA can lead to modest yet biologically significant increases in LspA protein levels that confer a heteroresistance phenotype.

Clinical development of antibiotics with novel mechanisms of action to kill pathogenic bacteria is challenging, in part, due to the inevitable emergence of resistance. A phenomenon of potential clinical importance that is broadly overlooked in preclinical development is heteroresistance, an often-unstable phenotype in which subpopulations of bacterial cells show decreased antibiotic susceptibility relative to the dominant population. Here, we describe a new globomycin analog, G0790, with potent activity against the Escherichia coli type II signal peptidase LspA and uncover two novel resistance mechanisms to G0790 in the clinical uropathogenic E. coli strain CFT073. Building on the previous finding that complete deletion of Lpp, the major Gram-negative outer membrane lipoprotein, leads to globomycin resistance, we also find that an unexpectedly modest decrease in Lpp levels mediated by insertion-based disruption of regulatory elements is sufficient to confer G0790 resistance and increase sensitivity to serum killing. In addition, we describe a heteroresistance phenotype mediated by genomic amplifications of lspA that result in increased LspA levels sufficient to overcome inhibition by G0790 in culture. These genomic amplifications are highly unstable and are lost after as few as two subcultures in the absence of G0790, which places amplification-containing resistant strains at high risk of being misclassified as susceptible by routine antimicrobial susceptibility testing. In summary, our study uncovers two vastly different mechanisms of resistance to LspA inhibitors in E. coli and emphasizes the importance of considering the potential impact of unstable and heterogenous phenotypes when developing antibiotics for clinical use.

An Atypical AAA+ ATPase Assembly Controls Efficient Transposition through DNA Remodeling and Transposase Recruitment

Transposons are ubiquitous genetic elements that drive genome rearrangements, evolution, and the spread of infectious disease and drug-resistance. Many transposons, such as Mu, Tn7, and IS21, require regulatory AAA+ ATPases for function. We use X-ray crystallography and cryo-electron microscopy to show that the ATPase subunit of IS21, IstB, assembles into a clamshell-shaped decamer that sandwiches DNA between two helical pentamers of ATP-associated AAA+ domains, sharply bending the duplex into a 180° U-turn. Biochemical studies corroborate key features of the structure and further show that the IS21 transposase, IstA, recognizes the IstB•DNA complex and promotes its disassembly by stimulating ATP hydrolysis. Collectively, these studies reveal a distinct manner of higher-order assembly and client engagement by a AAA+ ATPase and suggest a mechanistic model where IstB binding and subsequent DNA bending primes a selected insertion site for efficient transposition.

Soluble expression, purification and characterization of the full length IS2 Transposase

BACKGROUND

The two-step transposition pathway of insertion sequences of the IS3 family, and several other families, involves first the formation of a branched figure-of-eight (F-8) structure by an asymmetric single strand cleavage at one optional donor end and joining to the flanking host DNA near the target end. Its conversion to a double stranded minicircle precedes the second insertional step, where both ends function as donors. In IS2, the left end which lacks donor function in Step I acquires it in Step II. The assembly of two intrinsically different protein-DNA complexes in these F-8 generating elements has been intuitively proposed, but a barrier to testing this hypothesis has been the difficulty of isolating a full length, soluble and active transposase that creates fully formed synaptic complexes in vitro with protein bound to both binding and catalytic domains of the ends. We address here a solution to expressing, purifying and structurally analyzing such a protein.

RESULTS

A soluble and active IS2 transposase derivative with GFP fused to its C-terminus functions as efficiently as the native protein in in vivo transposition assays. In vitro electrophoretic mobility shift assay data show that the partially purified protein prepared under native conditions binds very efficiently to cognate DNA, utilizing both N- and C-terminal residues. As a precursor to biophysical analyses of these complexes, a fluorescence-based random mutagenesis protocol was developed that enabled a structure-function analysis of the protein with good resolution at the secondary structure level. The results extend previous structure-function work on IS3 family transposases, identifying the binding domain as a three helix H + HTH bundle and explaining the function of an atypical leucine zipper-like motif in IS2. In addition gain- and loss-of-function mutations in the catalytic active site define its role in regional and global binding and identify functional signatures that are common to the three dimensional catalytic core motif of the retroviral integrase superfamily.

CONCLUSIONS

Intractably insoluble transposases, such as the IS2 transposase, prepared by solubilization protocols are often refractory to whole protein structure-function studies. The results described here have validated the use of GFP-tagging and fluorescence-based random mutagenesis in overcoming this limitation at the secondary structure level.

A model for the molecular organisation of the IS911 transpososome

Tight regulation of transposition activity is essential to limit damage transposons may cause by generating potentially lethal DNA rearrangements. Assembly of a bona fide protein-DNA complex, the transpososome, within which transposition is catalysed, is a crucial checkpoint in this regulation. In the case of IS911, a member of the large IS3 bacterial insertion sequence family, the transpososome (synaptic complex A; SCA) is composed of the right and left inverted repeated DNA sequences (IRR and IRL) bridged by the transposase, OrfAB (the IS911-encoded enzyme that catalyses transposition). To characterise further this important protein-DNA complex in vitro, we used different tagged and/or truncated transposase forms and analysed their interaction with IS911 ends using gel electrophoresis. Our results allow us to propose a model in which SCA is assembled with a dimeric form of the transposase. Furthermore, we present atomic force microscopy results showing that the terminal inverted repeat sequences are probably assembled in a parallel configuration within the SCA. These results represent the first step in the structural description of the IS911 transpososome, and are discussed in comparison with the very few other transpososome examples described in the literature.

IS21-558 insertion sequences are involved in the mobility of the multiresistance gene cfr

During a study of florfenicol-resistant porcine staphylococci from Denmark, the genes cfr and fexA were detected in the chromosomal DNA or on plasmids of Staphylococcus hyicus, Staphylococcus warneri, and Staphylococcus simulans. A novel variant of the phenicol resistance transposon Tn558 was detected on the ca. 43-kb plasmid pSCFS6 in S. warneri and S. simulans isolates. Sequence analysis of a 22,010-bp segment revealed that the new Tn558 variant harbored an additional resistance gene region integrated into the tnpC reading frame. This resistance gene region consisted of the clindamycin exporter gene lsa(B) and the gene cfr for combined resistance to phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin A antibiotics bracketed by IS21-558 insertion sequences orientated in the same direction. A 6-bp target site duplication was detected at the integration site within tnpC. Transpositionally active forms of the IS21-558 element, known as minicircles, were detected by PCR and suggest that this insertion sequence is involved in the mobility of the multiresistance gene cfr. Based on the knowledge of the transposition pathways of IS21-like insertion sequences and the sequence features detected, the resistance gene region of plasmid pSCFS6 is believed to have developed via IS21-558-mediated cointegrate formation. The data obtained in this study identified the multiresistance gene cfr not only in three novel host species but also in a novel genetic context whose further analysis suggested that insertion sequences of the type IS21-558 are likely to be involved in the dissemination of cfr.

Characterization of a Mycobacterium bovis BCG insertion sequence related to the IS21 family

The structure and distribution of a Mycobacterium bovis BCG insertion element of the IS21 family were investigated. Several IS21-like elements found in mycobacterial genomes were separated in four types, following their nucleic acid similarities. The M. bovis BCG IS21 element is highly similar to IS1533 (class I), 70% similar to IS1534 (class II), 52% similar to IS1532 (class III) of Mycobacterium tuberculosis, and 54% similar to both an Mycobacterium avium serovar 2 and an M. avium silvaticum IS (class IV). The M. bovis BCG IS21 element of the class I appears to be present in a single copy in the genome of M. bovis BCG, M. bovis, M. tuberculosis and Mycobacterium africanum and to be absent from all other tested species of the Corynebacteria-Mycobacteria-Nocardia group.