Continuous Electrospray Ionization Mass Spectrometry Assay for Measuring Phospholipase Activity against Liposomes.

Phospholipases have diverse roles in lipid and cell membrane biology. In animal venoms, they can have roles as neurotoxins or myotoxins that disrupt the integrity of cell membranes. In this work, we describe a temperature-controlled, continuous electrospray ionization mass spectrometry (ESI-MS) assay for measuring phospholipase A2 activity against liposomes. The enzyme used in this assay was paradoxin, which is a neurotoxic trimeric phospholipase A2 from inland taipan snake venom. Previously developed ESI-MS-based phospholipase assays have been discontinuous and analyzed hydrolysis of single lipid molecules by liquid chromatography ESI-MS. In this work, a continuous assay was developed against liposomes, a more complex substrate that more closely reflects the natural substrate for paradoxin. The assay confirmed the requirement for Ca2+ and allowed measurement of Michaelis-Menten-type parameters. The use of ESI-MS for lipid detection enabled nuanced insights into the effect of changing assay conditions not only on the enzyme but also on the liposome substrate. Changing the metal ion concentrations did not significantly change the liposomes but did affect enzymatic activity. Increasing temperature did not substantially affect the secondary structure of paradoxin but affected liposome size, resulting in increased enzymatic activity consistent with the disruption of the phosphatidylcholine membrane, increasing accessibility of sn-2 ester bonds. The continuous ESI-MS method described herein can be applied to other enzyme reactions, particularly those which utilize complex lipid substrates.

A mechanism for the extension and unfolding of parallel telomeric G-quadruplexes by human telomerase at single-molecule resolution.

Telomeric G-quadruplexes (G4) were long believed to form a protective structure at telomeres, preventing their extension by the ribonucleoprotein telomerase. Contrary to this belief, we have previously demonstrated that parallel-stranded conformations of telomeric G4 can be extended by human and ciliate telomerase. However, a mechanistic understanding of the interaction of telomerase with structured DNA remained elusive. Here, we use single-molecule fluorescence resonance energy transfer (smFRET) microscopy and bulk-phase enzymology to propose a mechanism for the resolution and extension of parallel G4 by telomerase. Binding is initiated by the RNA template of telomerase interacting with the G-quadruplex; nucleotide addition then proceeds to the end of the RNA template. It is only through the large conformational change of translocation following synthesis that the G-quadruplex structure is completely unfolded to a linear product. Surprisingly, parallel G4 stabilization with either small molecule ligands or by chemical modification does not always inhibit G4 unfolding and extension by telomerase. These data reveal that telomerase is a parallel G-quadruplex resolvase.

A new class of quadruplex DNA-binding nickel Schiff base complexes.

We have prepared six new nickel Schiff base complexes via reactions of substituted benzophenones with different diamines in the presence of nickel(ii). These new complexes were then reacted with 1-(2-choroethyl)piperidine to afford a further six novel nickel(ii) Schiff base complexes bearing pendant ethylpiperidine groups. The complexes bearing the ethylpiperidine moieties had greater solubility in water, and were therefore suitable for use in DNA binding experiments. ESI mass spectra of solutions containing 4 and the parallel, tetramolecular quadruplex Q4, contained ions attributable to formation of non-covalent complexes. In contrast, no ions from non-covalent complexes were observed when the experiments were repeated using 4 and either a double stranded DNA (dsDNA) molecule (D2), or parallel Q1, a unimolecular quadruplex DNA (qDNA). The ESI-MS binding study also revealed that 14 has a significant ability to form non-covalent complexes with qDNA, but does not interact to the same extent with D2. This is supported by the large changes to the ellipticity of bands observed in the circular dichroism spectra of two different unimolecular qDNA molecules (c-kit1 and Q1), including the latter annealed under conditions designed to induce formation of alternative topologies (antiparallel and hybrid). In Fluorescent Indicator Displacement (FID) assays conducted using the new nickel complexes, 14 gave the lowest values of DC50 for experiments conducted with Q1 and Q4. Furthermore, 14 showed greater stabilisation of an antiparallel qDNA molecule in FRET assays than when the other new complexes were examined. These results highlight the potential of 14 as a lead complex for future structure/DNA binding investigations. This is reinforced by the results obtained from cytotoxicity studies performed using four of the nickel complexes, including 14, and Chinese hamster lung cancer (V79) cells, which gave IC50 values between 4 and 12 µM. These complexes were also shown to have the ability to induce apoptosis in the same cancer cell line.

A study of Pt(II)-phenanthroline complex interactions with double-stranded and G-quadruplex DNA by ESI-MS, circular dichroism, and computational docking.

The binding interactions of a series of square-planar platinum(II)-phenanthroline complexes of the type [Pt(PL)(AL)]2+ [where PL = variously methyl-substituted 1,10-phenanthroline (phen) and AL = ethane-1,2-diamine (en)] were assessed with a G-quadruplex DNA (5'-TTG GGG GT-3', G4DNA) and a double-stranded DNA (5'-CGC GAA TTC GCG-3', dsDNA) sequence by ESI-MS. The results indicate a strong correlation between G4DNA affinity and increasing phenanthroline methyl substitution. Circular dichroism (CD) spectroscopy and molecular docking studies also support the finding that increased substitution of the phenanthroline ligand increased selectivity for G4DNA. ESI-MS was used to probe the interaction of a range of square-planar Pt(II)-phenanthroline complexes with double-stranded and G-quadruplex DNA.

Conditions for Analysis of Native Protein Structures Using Uniform Field Drift Tube Ion Mobility Mass Spectrometry and Characterization of Stable Calibrants for TWIM-MS.

Determination of collisional cross sections (CCS) by travelling wave ion mobility mass spectrometry (TWIM-MS) requires calibration against standards for which the CCS has been measured previously by drift tube ion mobility mass spectrometry (DTIM-MS). The different extents of collisional activation in TWIM-MS and DTIM-MS can give rise to discrepancies in the CCS of calibrants across the two platforms. Furthermore, the conditions required to ionize and transmit large, folded proteins and assemblies may variably affect the structure of the calibrants and analytes. Stable hetero-oligomeric phospholipase A2 (PDx) and its subunits were characterized as calibrants for TWIM-MS. Conditions for acquisition of native-like TWIM (Synapt G1 HDMS) and DTIM (Agilent 6560 IM-Q-TOF) mass spectra were optimized to ensure the spectra exhibited similar charge state distributions. CCS measurements (DTIM-MS) for ubiquitin, cytochrome c, holo-myoglobin, serum albumin and glutamate dehydrogenase were in good agreement with other recent results determined using this and other DTIM-MS instruments. PDx and its ß and γ subunits were stable across a wide range of cone and trap voltages in TWIM-MS and were stable in the presence of organic solvents. The CCS of PDx and its subunits were determined by DTIM-MS and were used as calibrants in determination of CCS of native-like cytochrome c, holo-myoglobin, carbonic anhydrase, serum albumin and haemoglobin in TWIM-MS. The CCS values were in good agreement with those measured by DTIM-MS where available. These experiments demonstrate conditions for analysis of native-like proteins using a commercially available DTIM-MS instrument, characterize robust calibrants for TWIM-MS, and present CCS values determined by DTIM-MS and TWIM-MS for native proteins to add to the current literature database. Graphical Abstract ᅟ.

BSD2 is a Rubisco-specific assembly chaperone, forms intermediary hetero-oligomeric complexes, and is nonlimiting to growth in tobacco.

The folding and assembly of Rubisco large and small subunits into L8 S8 holoenzyme in chloroplasts involves many auxiliary factors, including the chaperone BSD2. Here we identify apparent intermediary Rubisco-BSD2 assembly complexes in the model C3 plant tobacco. We show BSD2 and Rubisco content decrease in tandem with leaf age with approximately half of the BSD2 in young leaves (~70 nmol BSD2 protomer.m2 ) stably integrated in putative intermediary Rubisco complexes that account for <0.2% of the L8 S8 pool. RNAi-silencing BSD2 production in transplastomic tobacco producing bacterial L2 Rubisco had no effect on leaf photosynthesis, cell ultrastructure, or plant growth. Genetic crossing the same RNAi-bsd2 alleles into wild-type tobacco however impaired L8 S8 Rubisco production and plant growth, indicating the only critical function of BSD2 is in Rubisco biogenesis. Agrobacterium mediated transient expression of tobacco, Arabidopsis, or maize BSD2 reinstated Rubisco biogenesis in BSD2-silenced tobacco. Overexpressing BSD2 in tobacco chloroplasts however did not alter Rubisco content, activation status, leaf photosynthesis rate, or plant growth in the field or in the glasshouse at 20°C or 35°C. Our findings indicate BSD2 functions exclusively in Rubisco biogenesis, can efficiently facilitate heterologous plant Rubisco assembly, and is produced in amounts nonlimiting to tobacco growth.

Crystal structures and biochemical characterization of DNA sliding clamps from three Gram-negative bacterial pathogens.

Bacterial sliding clamps bind to DNA and act as protein-protein interaction hubs for several proteins involved in DNA replication and repair. The partner proteins all bind to a common pocket on sliding clamps via conserved linear peptide sequence motifs, which suggest the pocket as an attractive target for development of new antibiotics. Herein we report the X-ray crystal structures and biochemical characterization of ß sliding clamps from the Gram-negative pathogens Pseudomonas aeruginosa, Acinetobacter baumannii and Enterobacter cloacae. The structures reveal close similarity between the pathogen and Escherichia coli clamps and similar patterns of binding to linear clamp-binding motif peptides. The results suggest that linear motif-sliding clamp interactions are well conserved and an antibiotic targeting the sliding clamp should have broad-spectrum activity against Gram-negative pathogens.

Effect of structure variations on the quadruplex DNA binding ability of nickel Schiff base complexes.

Two different series of nickel Schiff base complexes were prepared as part of a study aimed at discovering new compounds with high affinity and selectivity for quadruplex DNA (qDNA). The new complexes were prepared by modification of a literature method for synthesising N,N'-bis-(4-((1-(2-ethyl)piperidine)-oxy)salicylidene)phenylenediaminenickel(ii) (complex (1)). For Series 1 complexes, the phenylenediamine head group of the literature complex was replaced with ethylenediamine, phenanthrenediamine, R,R- and S,S-diaminocyclohexane. These complexes, as well as an asymmetric molecule featuring a naphthalene moiety on one side and a single ethyl piperidinyl salicylidene group on the other, were prepared in order to examine the effect of varying the number and position of aromatic groups on DNA binding. Series 2 complexes were isomers of those in Series 1, in which pendant ethyl piperidine groups were located at different positions. All new complexes were characterised by 1D and 2D NMR spectroscopic methods alongside microanalysis and ESI-MS. In addition, the solid state structures of eight new complexes were determined using single crystal X-ray diffraction methods. N,N'-Bis-(4-((1-(2-ethyl)piperidine)oxy)-salicylidine)diaminophenanthrenenickel(ii) (9), was shown by ESI-MS, CD spectroscopy and UV melting studies to exhibit a greater affinity towards, and ability to stabilise, dsDNA than all other complexes in the first series. ESI-MS revealed (9) to have a strong tendency to form a 1 : 1 complex with the tetramolecular, parallel qDNA molecule Q4, however it exhibited low affinity towards the parallel unimolecular qDNA molecule Q1. The enantiomeric complexes (5) and (7), featuring R,R- and S,S-diaminocyclohexane moieties, respectively, showed similar binding profiles towards all DNA molecules investigated, whereas the asymmetric complex (11), exhibited very low DNA affinity in all cases. Series 2 complexes showed very similar DNA affinity and selectivity to their isomeric counterparts in Series 1. For example, (14) and (15), both of which contain a phenylenediamine head group, showed higher affinity towards D2, Q1 and Q4, than any of the other Series 2 complexes. In addition, complex (21), which contains a meso-1,2-diphenylethylenediamine moiety, interacted strongly with Q4, but not D2 or Q1. This observation was very similar to that made previously for the isomeric complex (3).

Telomeric G-quadruplexes are a substrate and site of localization for human telomerase.

It has been hypothesized that G-quadruplexes can sequester the 3' end of the telomere and prevent it from being extended by telomerase. Here we purify and characterize stable, conformationally homogenous human telomeric G-quadruplexes, and demonstrate that human telomerase is able to extend parallel, intermolecular conformations in vitro. These G-quadruplexes align correctly with the RNA template of telomerase, demonstrating that at least partial G-quadruplex resolution is required. A highly purified preparation of human telomerase retains this extension ability, establishing that the core telomerase enzyme complex is sufficient for partial G-quadruplex resolution and extension. The parallel-specific G-quadruplex ligand N-methyl mesoporphyrin IX (NMM) causes an increase in telomeric G-quadruplexes, and we show that telomerase colocalizes with a subset of telomeric G-quadruplexes in vivo. The ability of telomerase to partially unwind, extend and localize to these structures implies that parallel telomeric G-quadruplexes may play an important biological role.

Bacterial Sliding Clamp Inhibitors that Mimic the Sequential Binding Mechanism of Endogenous Linear Motifs.

The bacterial DNA replication machinery presents new targets for the development of antibiotics acting via novel mechanisms. One such target is the protein-protein interaction between the DNA sliding clamp and the conserved peptide linear motifs in DNA polymerases. We previously established that binding of linear motifs to the Escherichia coli sliding clamp occurs via a sequential mechanism that involves two subsites (I and II). Here, we report the development of small-molecule inhibitors that mimic this mechanism. The compounds contain tetrahydrocarbazole moieties as "anchors" to occupy subsite I. Functional groups appended at the tetrahydrocarbazole nitrogen bind to a channel gated by the side chain of M362 and lie at the edge of subsite II. One derivative induced the formation of a new binding pocket, termed subsite III, by rearrangement of a loop adjacent to subsite I. Discovery of the extended binding area will guide further inhibitor development.

ESI-MS Investigation of an Equilibrium between a Bimolecular Quadruplex DNA and a Duplex DNA/RNA Hybrid.

Electrospray ionization mass spectrometry (ESI-MS) conditions were optimized for simultaneous observation of a bimolecular qDNA and a Watson-Crick base-paired duplex DNA/RNA hybrid. The DNA sequence used was telomeric DNA, and the RNA contained the template for telomerase-mediated telomeric DNA synthesis. Addition of RNA to the quadruplex DNA (qDNA) resulted in formation of the duplex DNA/RNA hybrid. Melting profiles obtained using circular dichroism spectroscopy confirmed that the DNA/RNA hybrid exhibited greater thermal stability than the bimolecular qDNA in solution. Binding of a 13-substituted berberine (1) derivative to the bimolecular qDNA stabilized its structure as evidenced by an increase in its stability in the mass spectrometer, and an increase in its circular dichroism (CD) melting temperature of 10°C. The DNA/RNA hybrid did not bind the ligand extensively and its thermal stability was unchanged in the presence of (1). The qDNA-ligand complex resisted unfolding in the presence of excess RNA, limiting the formation of the DNA/RNA hybrid. Previously, it has been proposed that DNA secondary structures, such as qDNA, may be involved in the telomerase mechanism. DNA/RNA hybrid structures occur at the active site of telomerase. The results presented in the current work show that if telomeric DNA was folded into a qDNA structure, it is possible for a DNA/RNA hybrid to form as is required during template alignment. The discrimination of ligand (1) for binding to the bimolecular qDNA over the DNA/RNA hybrid positions it as a useful compound for probing the role(s), if any, of antiparallel qDNA in the telomerase mechanism.

Improving recombinant Rubisco biogenesis, plant photosynthesis and growth by coexpressing its ancillary RAF1 chaperone.

Enabling improvements to crop yield and resource use by enhancing the catalysis of the photosynthetic CO2-fixing enzyme Rubisco has been a longstanding challenge. Efforts toward realization of this goal have been greatly assisted by advances in understanding the complexities of Rubisco's biogenesis in plastids and the development of tailored chloroplast transformation tools. Here we generate transplastomic tobacco genotypes expressing Arabidopsis Rubisco large subunits (AtL), both on their own (producing tob(AtL) plants) and with a cognate Rubisco accumulation factor 1 (AtRAF1) chaperone (producing tob(AtL-R1) plants) that has undergone parallel functional coevolution with AtL. We show AtRAF1 assembles as a dimer and is produced in tob(AtL-R1) and Arabidopsis leaves at 10-15 nmol AtRAF1 monomers per square meter. Consistent with a postchaperonin large (L)-subunit assembly role, the AtRAF1 facilitated two to threefold improvements in the amount and biogenesis rate of hybrid L8(A)S8(t) Rubisco [comprising AtL and tobacco small (S) subunits] in tob(AtL-R1) leaves compared with tob(AtL), despite >threefold lower steady-state Rubisco mRNA levels in tob(AtL-R1). Accompanying twofold increases in photosynthetic CO2-assimilation rate and plant growth were measured for tob(AtL-R1) lines. These findings highlight the importance of ancillary protein complementarity during Rubisco biogenesis in plastids, the possible constraints this has imposed on Rubisco adaptive evolution, and the likely need for such interaction specificity to be considered when optimizing recombinant Rubisco bioengineering in plants.

Synthesis and characterisation of nickel Schiff base complexes containing the meso-1,2-diphenylethylenediamine moiety: selective interactions with a tetramolecular DNA quadruplex.

As part of a program of preparing metal complexes which exhibit unique affinities towards different DNA structures, we have synthesised the novel Schiff base complex N,N'-bis-4-(hydroxysalicylidine)meso-diphenylethylenediaminenickel(ii) (), via the reaction of meso-1,2-diphenylethylenediamine and 2,4-dihydroxybenzaldehyde. This compound was subsequently reacted with 1-(2-chloroethyl)piperidine or 1-(2-chloropropyl)piperidine, to afford the alkylated complexes N,N'-bis-(4-((1-(2-ethyl)piperidine)oxy)salicylidine)meso-1,2-diphenylethylenediaminenickel(ii) () and N,N'-bis-(4-((1-(3-propyl)piperidine)oxy)-salicylidine)meso-1,2-diphenylethylenediaminenickel(ii) (), respectively. These complexes were characterised by microanalysis and X-ray crystallography in the solid state, and in solution by (1)H and (13)C NMR spectroscopy. Electrospray ionisation mass spectrometry (ESI-MS) was used to confirm the identity of () and (). The affinities of () and () towards a discrete 16 mer duplex DNA molecule, and examples of both tetramolecular and unimolecular DNA quadruplexes, was explored using a variety of techniques. In addition, the affinity of two other complexes () and (), towards the same DNA molecules was examined. Complexes () and () were prepared by methods analogous to those which afforded () and (), however 1,2-phenylenediamine was used instead of meso-1,2-diphenylethylenediamine in the initial step of the synthetic procedure. The results of ESI-MS and DNA melting temperature measurements suggest that () and () exhibit a lower affinity than () and () towards the 16 mer duplex DNA molecule, while circular dichroism (CD) spectroscopy suggested that none of the four complexes had a major effect on the conformation of the nucleic acid. In contrast, ESI-MS and CD spectroscopy suggested that both () and () show significant binding to a tetramolecular DNA quadruplex. The results of ESI-MS and Fluorescence Resonance Energy Transfer (FRET) assays indicated that () and () did not bind as tightly to a unimolecular DNA quadruplex, although both complexes had a major effect on the CD spectrum of the latter. These results highlight that the presence of the meso-1,2-diphenylethylenediamine moiety in metal complexes of this type may provide a general method for instilling selectivity for some DNA quadruplexes over dsDNA.

An investigation into the interactions of gold nanoparticles and anti-arthritic drugs with macrophages, and their reactivity towards thioredoxin reductase.

Gold(I) complexes are an important tool in the arsenal of established approaches for treating rheumatoid arthritis (RA), while some recent studies have suggested that gold nanoparticles (Au NPs) may also be therapeutically efficacious. These observations prompted the current biological studies involving gold(I) anti-RA agents and Au NPs, which are aimed towards improving our knowledge of how they work. The cytotoxicity of auranofin, aurothiomalate, aurothiosulfate and Au NPs towards RAW264.7 macrophages was evaluated using the MTT assay, with the former compound proving to be the most toxic. The extent of cellular uptake of the various gold agents was determined using graphite furnace atomic absorption spectrometry, while their distribution within macrophages was examined using microprobe synchrotron radiation X-ray fluorescence spectroscopy. The latter technique showed accumulation of gold in discrete regions of the cell, and co-localisation with sulfur in the case of cells treated with aurothiomalate or auranofin. Electrospray ionization mass spectrometry was used to characterize thioredoxin reductase (TrxR) in which the penultimate selenocysteine residue was replaced by cysteine. Mass spectra of solutions of TrxR and aurothiomalate, aurothiosulfate or auranofin showed complexes containing bare gold atoms bound to the protein, or protein adducts containing gold atoms retaining some of their initial ligands. These results support TrxR being an important target of gold(I) drugs used to treat RA, while the finding that Au NPs are incorporated into macrophages, but elicit little toxicity, indicates further exploration of their potential for treatment of RA is warranted.

DNA replication is the target for the antibacterial effects of nonsteroidal anti-inflammatory drugs.

Evidence suggests that some nonsteroidal anti-inflammatory drugs (NSAIDs) possess antibacterial properties with an unknown mechanism. We describe the in vitro antibacterial properties of the NSAIDs carprofen, bromfenac, and vedaprofen, and show that these NSAIDs inhibit the Escherichia coli DNA polymerase III ß subunit, an essential interaction hub that acts as a mobile tether on DNA for many essential partner proteins in DNA replication and repair. Crystal structures show that the three NSAIDs bind to the sliding clamp at a common binding site required for partner binding. Inhibition of interaction of the clamp loader and/or the replicative polymerase α subunit with the sliding clamp is demonstrated using an in vitro DNA replication assay. NSAIDs thus present promising lead scaffolds for novel antibacterial agents targeting the sliding clamp.

Discovery of lead compounds targeting the bacterial sliding clamp using a fragment-based approach.

The bacterial sliding clamp (SC), also known as the DNA polymerase III ß subunit, is an emerging antibacterial target that plays a central role in DNA replication, serving as a protein-protein interaction hub with a common binding pocket to recognize linear motifs in the partner proteins. Here, fragment-based screening using X-ray crystallography produced four hits bound in the linear-motif-binding pocket of the Escherichia coli SC. Compounds structurally related to the hits were identified that inhibited the E. coli SC and SC-mediated DNA replication in vitro. A tetrahydrocarbazole derivative emerged as a promising lead whose methyl and ethyl ester prodrug forms showed minimum inhibitory concentrations in the range of 21-43 µg/mL against representative Gram-negative and Gram-positive bacteria species. The work demonstrates the utility of a fragment-based approach for identifying bacterial sliding clamp inhibitors as lead compounds with broad-spectrum antibacterial activity.

Structural and thermodynamic dissection of linear motif recognition by the E. coli sliding clamp.

Protein-protein interactions based on linear motif (LM) recognition play roles in many cell regulatory processes. The E. coli sliding clamp is a protein mediator of replisome formation, which uses a common surface pocket composed of two subsites (I and II) to interact with LMs in multiple binding partners. A structural and thermodynamic dissection of sliding clamp-LM recognition has been performed, providing support for a sequential binding model. According to the model, a hydrophobic C-terminal LM dipeptide submotif acts as an anchor to establish initial contacts within subsite I, and this is followed by formation of a stabilizing hydrogen-bonding network between the flanking LM residues and subsite II. Differential solvation/desolvation during positioning of the submotifs is proposed as a driver for the sequential binding. Our model provides general insights into linear motif recognition and should guide the design of small-molecule inhibitors of the E. coli sliding clamp, an emerging antibacterial target.

A direct proofreader-clamp interaction stabilizes the Pol III replicase in the polymerization mode.

Processive DNA synthesis by the αεθ core of the Escherichia coli Pol III replicase requires it to be bound to the ß2 clamp via a site in the α polymerase subunit. How the ε proofreading exonuclease subunit influences DNA synthesis by α was not previously understood. In this work, bulk assays of DNA replication were used to uncover a non-proofreading activity of ε. Combination of mutagenesis with biophysical studies and single-molecule leading-strand replication assays traced this activity to a novel ß-binding site in ε that, in conjunction with the site in α, maintains a closed state of the αεθ-ß2 replicase in the polymerization mode of DNA synthesis. The ε-ß interaction, selected during evolution to be weak and thus suited for transient disruption to enable access of alternate polymerases and other clamp binding proteins, therefore makes an important contribution to the network of protein-protein interactions that finely tune stability of the replicase on the DNA template in its various conformational states.

Escherichia coli single-stranded DNA-binding protein: nanoESI-MS studies of salt-modulated subunit exchange and DNA binding transactions.

Single-stranded DNA-binding proteins (SSBs) are ubiquitous oligomeric proteins that bind with very high affinity to single-stranded DNA and have a variety of essential roles in DNA metabolism. Nanoelectrospray ionization mass spectrometry (nanoESI-MS) was used to monitor subunit exchange in full-length and truncated forms of the homotetrameric SSB from Escherichia coli. Subunit exchange in the native protein was found to occur slowly over a period of hours, but was significantly more rapid in a truncated variant of SSB from which the eight C-terminal residues were deleted. This effect is proposed to result from C-terminus mediated stabilization of the SSB tetramer, in which the C-termini interact with the DNA-binding cores of adjacent subunits. NanoESI-MS was also used to examine DNA binding to the SSB tetramer. Binding of single-stranded oligonucleotides [one molecule of (dT)(70), one molecule of (dT)(35), or two molecules of (dT)(35)] was found to prevent SSB subunit exchange. Transfer of SSB tetramers between discrete oligonucleotides was also observed and is consistent with predictions from solution-phase studies, suggesting that SSB-DNA complexes can be reliably analyzed by ESI mass spectrometry.