Characterization of a single-stranded DNA-binding protein from Pseudomonas aeruginosa PAO1

Anti-Skin Aging Potential, Antibacterial Activity, Inhibition of Single-Stranded DNA-Binding Protein, and Cytotoxic Effects of Acetone-Extracted Passiflora edulis (Tainung No. 1) Rind Extract on Oral Carcinoma Cells

The passion fruit, Passiflora edulis, recognized for its rich nutritional properties, has long been used for its varied ethnobotanical applications. This study investigates the therapeutic potential of P. edulis var. Tainung No. 1 rind extracts by examining their polyphenolic content (TPC), total flavonoid content (TFC), anti-skin aging activities against key enzymes such as elastase, tyrosinase, and hyaluronidase, and their ability to inhibit bacterial growth, single-stranded DNA-binding protein (SSB), and their cytotoxic effects on oral carcinoma cells. The acetone extract from the rind exhibited the highest levels of TPC, TFC, anti-SSB, and antibacterial activities. The antibacterial effectiveness of the acetone-extracted rind was ranked as follows: Escherichia coli > Pseudomonas aeruginosa > Staphylococcus aureus. A titration curve for SSB inhibition showed an IC50 value of 313.2 μg/mL, indicating the potency of the acetone extract in inhibiting SSB. It also significantly reduced the activity of enzymes associated with skin aging, particularly tyrosinase, with a 54.5% inhibition at a concentration of 100 μg/mL. Gas chromatography-mass spectrometry (GC-MS) analysis tentatively identified several major bioactive compounds in the acetone extract, including stigmast-5-en-3-ol, vitamin E, palmitic acid, stigmasterol, linoleic acid, campesterol, and octadecanoic acid. Molecular docking studies suggested some of these compounds as potential inhibitors of tyrosinase and SSB. Furthermore, the extract demonstrated anticancer potential against Ca9-22 oral carcinoma cells by inhibiting cell survival, migration, and proliferation and inducing apoptosis. These results underscore the potential of P. edulis (Tainung No. 1) rind as a promising candidate for anti-skin aging, antibacterial, and anticancer applications, meriting further therapeutic investigation.

Crystal Structure of an SSB Protein from Salmonella enterica and Its Inhibition by Flavanonol Taxifolin

Single-stranded DNA (ssDNA)-binding proteins (SSBs) play a central role in cells by participating in DNA metabolism, including replication, repair, recombination, and replication fork restart. SSBs are essential for cell survival and thus an attractive target for potential anti-pathogen chemotherapy. In this study, we determined the crystal structure and examined the size of the ssDNA-binding site of an SSB from Salmonella enterica serovar Typhimurium LT2 (SeSSB), a ubiquitous opportunistic pathogen which is highly resistant to antibiotics. The crystal structure was solved at a resolution of 2.8 Å (PDB ID 7F25), indicating that the SeSSB monomer possesses an oligonucleotide/oligosaccharide-binding (OB) fold domain at its N-terminus and a flexible tail at its C-terminus. The core of the OB-fold in the SeSSB is made of a six-stranded β-barrel capped by an α-helix. The crystal structure of the SeSSB contained two monomers per asymmetric unit, which may indicate the formation of a dimer. However, the gel-filtration chromatography analysis showed that the SeSSB forms a tetramer in solution. Through an electrophoretic mobility shift analysis, we characterized the stoichiometry of the SeSSB complexed with a series of ssDNA dA homopolymers, and the size of the ssDNA-binding site was determined to be around 22 nt. We also found the flavanonol taxifolin, also known as dihydroquercetin, capable of inhibiting the ssDNA-binding activity of the SeSSB. Thus, this result extended the SSB interactome to include taxifolin, a natural product with a wide range of promising pharmacological activities.

A Complexed Crystal Structure of a Single-Stranded DNA-Binding Protein with Quercetin and the Structural Basis of Flavonol Inhibition Specificity

Single-stranded DNA (ssDNA)-binding protein (SSB) plays a crucial role in DNA replication, repair, and recombination as well as replication fork restarts. SSB is essential for cell survival and, thus, is an attractive target for potential antipathogen chemotherapy. Whether naturally occurring products can inhibit SSB remains unknown. In this study, the effect of the flavonols myricetin, quercetin, kaempferol, and galangin on the inhibition of Pseudomonas aeruginosa SSB (PaSSB) was investigated. Furthermore, SSB was identified as a novel quercetin-binding protein. Through an electrophoretic mobility shift analysis, myricetin could inhibit the ssDNA binding activity of PaSSB with an IC₅₀ of 2.8 ± 0.4 μM. The effect of quercetin, kaempferol, and galangin was insignificant. To elucidate the flavonol inhibition specificity, the crystal structure of PaSSB complexed with the non-inhibitor quercetin was solved using the molecular replacement method at a resolution of 2.3 Å (PDB entry 7VUM) and compared with a structure with the inhibitor myricetin (PDB entry 5YUN). Although myricetin and quercetin bound PaSSB at a similar site, their binding poses were different. Compared with myricetin, the aromatic ring of quercetin shifted by a distance of 4.9 Å and an angle of 31° for hydrogen bonding to the side chain of Asn108 in PaSSB. In addition, myricetin occupied and interacted with the ssDNA binding sites Lys7 and Glu80 in PaSSB whereas quercetin did not. This result might explain why myricetin could, but quercetin could not, strongly inhibit PaSSB. This molecular evidence reveals the flavonol inhibition specificity and also extends the interactomes of the natural anticancer products myricetin and quercetin to include the OB-fold protein SSB.

Characterization of the Chimeric PriB-SSBc Protein

PriB is a primosomal protein required for the replication fork restart in bacteria. Although PriB shares structural similarity with SSB, they bind ssDNA differently. SSB consists of an N-terminal ssDNA-binding/oligomerization domain (SSBn) and a flexible C-terminal protein–protein interaction domain (SSBc). Apparently, the largest difference in structure between PriB and SSB is the lack of SSBc in PriB. In this study, we produced the chimeric PriB-SSBc protein in which Klebsiella pneumoniae PriB (KpPriB) was fused with SSBc of K. pneumoniae SSB (KpSSB) to characterize the possible SSBc effects on PriB function. The crystal structure of KpSSB was solved at a resolution of 2.3 Å (PDB entry 7F2N) and revealed a novel 114-GGRQ-117 motif in SSBc that pre-occupies and interacts with the ssDNA-binding sites (Asn14, Lys74, and Gln77) in SSBn. As compared with the ssDNA-binding properties of KpPriB, KpSSB, and PriB-SSBc, we observed that SSBc could significantly enhance the ssDNA-binding affinity of PriB, change the binding behavior, and further stimulate the PriA activity (an initiator protein in the pre-primosomal step of DNA replication), but not the oligomerization state, of PriB. Based on these experimental results, we discuss reasons why the properties of PriB can be retrofitted when fusing with SSBc.

Characterization of an SSB–dT25 complex: structural insights into the S-shaped ssDNA binding conformation

Single-stranded DNA (ssDNA)-binding proteins (SSBs) play an important role in all DNA-dependent cellular processes, such as DNA replication, recombination, repair, and replication restart. The N-terminal domain of SSBs forms an oligonucleotide/oligosaccharide-binding (OB) fold for ssDNA binding. The SSB–dC35 complex structure has revealed how an Escherichia coli SSB (EcSSB) tetramer binds to 65-nucleotide (nt)-long ssDNA, namely, the (SSB)₆₅ binding mode. Knowledge on whether the ssDNA-binding mode for EcSSB is typical for all SSBs or is bacterial strain and length dependent is limited. Here, we studied the ssDNA-binding properties of a Pseudomonas aeruginosa SSB (PaSSB) and investigated its interaction mode through crystallographic analysis. The complex crystal structure containing a PaSSB tetramer with two ssDNA chains was solved at a resolution of 1.91 Å (PDB entry 6IRQ). Results revealed that each bound ssDNA dT25 adopts an S-shaped conformation. This binding mode, as shown by the complex structure of PaSSB, differs significantly from (SSB)₆₅. ssDNA-binding contributions from aromatic residues in PaSSB, except the contribution of Trp54, were not significant. Using electrophoretic mobility shift analysis, we characterized the stoichiometry of PaSSB complexed with a series of ssDNA homopolymers. The minimal length of ssDNA required for PaSSB tetramer binding and the size of the ssDNA-binding site were 25 and 29 nt, respectively. These observations through structure–function analysis suggested that only two OB folds rather than four OB folds in PaSSB are enough for the formation of a stable complex with ssDNA. The PaSSB noninteracting OB folds proposed here may allow sliding via reptation in a dynamic ssDNA binding process.

Single-stranded DNA (ssDNA)-binding proteins (SSBs) play an important role in all DNA-dependent cellular processes, such as DNA replication, recombination, repair, and replication restart.

SAAV2152 is a single-stranded DNA binding protein: the third SSB in Staphylococcus aureus

Single-stranded DNA-binding proteins (SSBs) play crucial roles in DNA replication, repair, and recombination. Unlike E. coli, which contains only one type of SSB (EcSSB), some bacteria have two paralogous SSBs, namely, SsbA and SsbB. In this study, we found the third SSB-like protein in Staphylococcus aureus, SAAV2152, which was designated as SaSsbC. SaSsbC is a protein of 131 amino acids and shares 38%, 36%, and 33% sequence identity to SaSsbB, SaSsbA, and EcSSB, respectively. Gene map analysis showed that unlike the E. coli ssb gene, which is adjacent to uvrA gene, the S. aureus ssb gene SAAV2152 is flanked by the putative SceD, the putative YwpF, and fabZ genes. A homology model showed that SaSsbC consists of the classic oligonucleotide/oligosaccharide-binding fold at the N-terminus. At the C-terminus, SaSsbC did not exhibit sequence similarity to that of EcSSB. Electrophoretic mobility shift analysis showed that SaSsbC formed a single complex with ssDNA of different lengths. Mutational analysis revealed that Tyr36, Tyr47, Phe53, and Tyr81 in SaSsbC are at positions that structurally correspond to the important residues of EcSSB for binding to ssDNA and are also critical for SaSsbC to bind ssDNA. Unlike EcSSB, which can stimulate EcPriA, SaSsbC did not affect the activity of SaPriA. In addition, SaSsbA inhibitor 9-methyl-2,3,7-trihydroxy-6-fluorone (NSC5426) could inhibit the ssDNA-binding activity of SaSsbC with IC₅₀ of 78 μM. In conclusion, this study has identified and characterized SAAV2152 as a kind of SSB, and further research can directly focus on determining its actual physiological role in S. aureus.

The glycine-rich flexible region in SSB is crucial for PriA stimulation

Single-stranded DNA-binding protein (SSB) is essential for all DNA-dependent cellular processes. The mechanism through which PriA helicase, an initiator protein in the DNA replication restart process, is stimulated by SSB in Escherichia coli (Ec) has been established. Nevertheless, whether or not PriA stimulated by SSB is conserved among Gram-negative bacteria remains unclear, and the SSB specificity for the stimulation effect on PriA is unknown. In this study, three similar SSBs from Klebsiella pneumoniae (KpSSB), Salmonella enterica (StSSB), and Pseudomonas aeruginosa (PaSSB) were used to analyze the stimulation effect. Two chimeric proteins, namely, KpSSBn-PaSSBc and KpSSBn-StSSBc, were also used. KpSSB, StSSB, and KpSSBn-StSSBc can stimulate KpPriA activity, but PaSSB and KpSSBn-PaSSBc cannot. The crystal structure of PaSSB solved at 2.04 Å resolution (PDB entry 5YUO) reveals the classic OB fold structure, similar to that of EcSSB. Comparison of SSBs through sequence analysis showed that the typical glycine-rich flexible region in PaSSB contains very few glycine residues. Through analyses of protein chimeragenesis, structure–sequence, and ATPase stimulation effects, we concluded that the inherent difference in the glycine-rich flexible region among SSB species is a determinant of PriA stimulation. Further research can directly focus on determining the type of glycine-rich hinge that can stimulate PriA and the reason why bacterial SSBs need to evolve different C-terminal domains during evolution.

Single-stranded DNA-binding protein (SSB) is essential for all DNA-dependent cellular processes.

Staphylococcus aureus single-stranded DNA-binding protein SsbA can bind but cannot stimulate PriA helicase

Single-stranded DNA-binding protein (SSB) and PriA helicase play important roles in bacterial DNA replication restart process. The mechanism by which PriA helicase is bound and stimulated by SSB in Escherichia coli (Ec) has been established, but information on this process in Gram-positive bacteria are limited. We characterized the properties of SSB from Staphylococcus aureus (SaSsbA, a counterpart of EcSSB) and analyzed its interaction with SaPriA. The gel filtration chromatography analysis of purified SaSsbA showed a stable tetramer in solution. The crystal structure of SaSsbA determined at 1.82 Å resolution (PDB entry 5XGT) reveals that the classic oligonucleotide/oligosaccharide-binding folds are formed in the N-terminal DNA-binding domain, but the entire C-terminal domain is disordered. Unlike EcSSB, which can stimulate EcPriA via a physical interaction between EcPriA and the C-terminus of EcSSB (SSB-Ct), SaSsbA does not affect the activity of SaPriA. We also found that SaPriA can be bound by SaSsbA, but not by SaSsbA-Ct. Although no effect was found with SaSsbA, SaPriA can be significantly stimulated by the Gram-negative Klebsiella pneumoniae SSB (KpSSB). In addition, we found that the conserved SSB-Ct binding site of KpPriA (Trp82, Tyr86, Lys370, Arg697, and Gln701) is not present in SaPriA. Arg697 in KpPriA is known to play a critical role in altering the SSB₃₅/SSB₆₅ distribution, but this corresponding residue in SaPriA is Glu767 instead, which has an opposite charge to Arg. SaPriA E767R mutant was constructed and analyzed; however, it still cannot be stimulated by SaSsbA. Finally, we found that the conserved MDFDDDIPF motif in the Gram-negative bacterial SSB is DISDDDLPF in SaSsbA, i.e., F172 in EcSSB and F168 in KpSSB is S161 in SaSsbA, not F. When acting with SaSsbA S161F mutant, the activity of SaPriA was dramatically enhanced elevenfold. Overall, the conserved binding sites, both in EcPriA and EcSSB, are not present in SaPriA and SaSsbA, thereby no stimulation occurs. Our observations through structure-sequence comparison and mutational analyses indicate that the case of EcPriA-EcSSB is not applicable to SaPriA-SaSsbA because of inherent differences among the species.

C-terminal domain swapping of SSB changes the size of the ssDNA binding site

Single-stranded DNA-binding protein (SSB) plays an important role in DNA metabolism, including DNA replication, repair, and recombination, and is therefore essential for cell survival. Bacterial SSB consists of an N-terminal ssDNA-binding/oligomerization domain and a flexible C-terminal protein-protein interaction domain. We characterized the ssDNA-binding properties of Klebsiella pneumoniae SSB (KpSSB), Salmonella enterica Serovar Typhimurium LT2 SSB (StSSB), Pseudomonas aeruginosa PAO1 SSB (PaSSB), and two chimeric KpSSB proteins, namely, KpSSBnStSSBc and KpSSBnPaSSBc. The C-terminal domain of StSSB or PaSSB was exchanged with that of KpSSB through protein chimeragenesis. By using the electrophoretic mobility shift assay, we characterized the stoichiometry of KpSSB, StSSB, PaSSB, KpSSBnStSSBc, and KpSSBnPaSSBc, complexed with a series of ssDNA homopolymers. The binding site sizes were determined to be 26 ± 2, 21 ± 2, 29 ± 2, 21 ± 2, and 29 ± 2 nucleotides (nt), respectively. Comparison of the binding site sizes of KpSSB, KpSSBnStSSBc, and KpSSBnPaSSBc showed that the C-terminal domain swapping of SSB changes the size of the binding site. Our observations suggest that not only the conserved N-terminal domain but also the C-terminal domain of SSB is an important determinant for ssDNA binding.

DnaT is a single-stranded DNA binding protein

DnaT is one of the replication restart primosomal proteins required for reinitiating chromosomal DNA replication in bacteria. In this study, we identified and characterized the single-stranded DNA (ssDNA)-binding properties of DnaT using electrophoretic mobility shift analysis (EMSA), bioinformatic tools and two deletion mutant proteins, namely, DnaT26-179 and DnaT42-179. ConSurf analysis indicated that the N-terminal region of DnaT is highly variable. The analysis of purified DnaT and the deletion mutant protein DnaT42-179 by gel filtration chromatography showed a stable trimer in solution, indicating that the N-terminal region, amino acid 1-41, is not crucial for the oligomerization of DnaT. Contrary to PriB, which forms a single complex with a series of ssDNA homopolymers, DnaT, DnaT26-179 and DnaT42-179 form distinct complexes with ssDNA of different lengths and the size of binding site of 26 ± 2 nucleotides (nt). Using bioinformatic programs (ps)(2) and the analysis of the positively charged/hydrophobic residue distribution, as well as the biophysical results in this study, we propose a binding model for the DnaT trimer-ssDNA complex, in which 25-nt-long ssDNA is tethered on the surface groove located in the highly conserved C-terminal domain of DnaT. These results constitute the first study regarding ssDNA-binding activity of DnaT. Consequently, a hand-off mechanism for primosome assembly was modified.

Characterization of a single-stranded DNA-binding protein from Klebsiella pneumoniae: mutation at either Arg73 or Ser76 causes a less cooperative complex on DNA

Single-stranded DNA-binding protein (SSB) plays an important role in DNA metabolism, such as in processes like DNA replication, repair and recombination, and is essential for cell survival. Here, we characterized the ssDNA-binding properties of Klebsiella pneumoniae SSB (KpSSB) by using fluorescence-quenching measurements, electrophoretic mobility shift analysis (EMSA) and site-directed mutagenesis. Analysis of purified KpSSB by gel-filtration chromatography showed a stable tetramer in solution. In fluorescence titrations, KpSSB bound to 25-40 nucleotides (nt) per tetramer depending on the salt concentration. Using EMSA, we characterized the stoichiometry of KpSSB complexed with a series of ssDNA homopolymers, and the size of the binding site was determined to be 26 ± 1 nt. Mutation at either Arg73 or Ser76 of KpSSB caused a less cooperative complex on DNA. Arg73 forms an intermolecular hydrogen bond with Ser76, and this appears to be a likely driving force that directs the self-assembly of SSB on DNA.

A single residue determines the cooperative binding property of a primosomal DNA replication protein, PriB, to single-stranded DNA

PriB is a primosomal protein required for re-initiation of replication in bacteria. We characterized and compared the DNA-binding properties of PriB from Salmonella enterica serovar Typhimurium LT2 (StPriB) and Escherichia coli (EcPriB). Only one residue of EcPriB, V6, was different in StPriB (replaced by A6). Previous structural information revealed that this residue is located on the putative dimer-dimer interface of PriB and is not involved in single-stranded DNA (ssDNA) binding. The cooperative binding mechanism of StPriB to DNA is, however, very different from that of EcPriB. Unlike EcPriB, which forms a single complex with ssDNAs of various lengths, StPriB forms two or more distinct complexes. Based on these results, as well as information on structure, binding modes for forming a stable complex of PriB with ssDNA of 25 nucleotides (nt), (EcPriB)25, and (StPriB)25 are proposed.

Characterization of flavonol inhibition of DnaB helicase: real-time monitoring, structural modeling, and proposed mechanism

DnaB helicases are motor proteins essential for DNA replication, repair, and recombination and may be a promising target for developing new drugs for antibiotic-resistant bacteria. Previously, we established that flavonols significantly decreased the binding ability of Klebsiella pneumoniae DnaB helicase (KpDnaB) to dNTP. Here, we further investigated the effect of flavonols on the inhibition of the ssDNA binding, ATPase activity, and dsDNA-unwinding activity of KpDnaB. The ssDNA-stimulated ATPase activity of KpDnaB was decreased to 59%, 75%, 65%, and 57%, in the presence of myricetin, quercetin, kaempferol, and galangin, respectively. The ssDNA-binding activity of KpDnaB was only slightly decreased by flavonols. We used a continuous fluorescence assay, based on fluorescence resonance energy transfer (FRET), for real-time monitoring of KpDnaB helicase activity in the absence and presence of flavonols. Using this assay, the flavonol-mediated inhibition of the dsDNA-unwinding activity of KpDnaB was observed. Modeled structures of bound and unbound DNA showed flavonols binding to KpDnaB with distinct poses. In addition, these structural models indicated that L214 is a key residue in binding any flavonol. On the basis of these results, we proposed mechanisms for flavonol inhibition of DNA helicase. The resulting information may be useful in designing compounds that target K. pneumoniae and other bacterial DnaB helicases.

Crystal structure and DNA-binding mode of Klebsiella pneumoniae primosomal PriB protein

PriB is a primosomal DNA replication protein required for the re-initiation of replication in bacteria. In this study, we investigated the gene expression of PriB in Klebsiella pneumoniae (KpPriB) and characterized the gene product through crystal structural and functional analyses. Quantitative polymerase chain reaction analysis (Q-PCR) indicated that the 104-aa priB was expressed in K. pneumoniae with a C(T) value of 22.4. The crystal structure of KpPriB (Protein Data Bank entry: 4APV) determined at a resolution of 2.1 Å was similar to that of Escherichia coli PriB (EcPriB). KpPriB formed a single complex with single-stranded DNA (ssDNA) of different lengths, suggesting a highly cooperative process. Structure-based mutational analysis revealed that substitution at K18, F42, R44, W47, K82, K84, or K89 but not R34 in KpPriB had a significant effect on both ssDNA and double-stranded DNA (dsDNA) binding. Based on these findings, the known ssDNA interaction sites of PriB were expanded to include R44 and F42, thus allowing nucleic acids to wrap around the whole PriB protein.

Characterization of the single stranded DNA binding protein SsbB encoded in the Gonoccocal Genetic Island

Background

Most strains of Neisseria gonorrhoeae carry a Gonococcal Genetic Island which encodes a type IV secretion system involved in the secretion of ssDNA. We characterize the GGI-encoded ssDNA binding protein, SsbB. Close homologs of SsbB are located within a conserved genetic cluster found in genetic islands of different proteobacteria. This cluster encodes DNA-processing enzymes such as the ParA and ParB partitioning proteins, the TopB topoisomerase, and four conserved hypothetical proteins. The SsbB homologs found in these clusters form a family separated from other ssDNA binding proteins.

Methodology/Principal Findings

In contrast to most other SSBs, SsbB did not complement the Escherichia coli ssb deletion mutant. Purified SsbB forms a stable tetramer. Electrophoretic mobility shift assays and fluorescence titration assays, as well as atomic force microscopy demonstrate that SsbB binds ssDNA specifically with high affinity. SsbB binds single-stranded DNA with minimal binding frames for one or two SsbB tetramers of 15 and 70 nucleotides. The binding mode was independent of increasing Mg²⁺ or NaCl concentrations. No role of SsbB in ssDNA secretion or DNA uptake could be identified, but SsbB strongly stimulated Topoisomerase I activity.

Conclusions/Significance

We propose that these novel SsbBs play an unknown role in the maintenance of genetic islands.

Biochemical characterization of allantoinase from Escherichia coli BL21

Bacterial allantoinase (ALLase; EC 3.5.2.5), which catalyzes the conversion of allantoin into allantoate, possesses a binuclear metal center in which two metal ions are bridged by a posttranslationally carboxylated lysine. Here, we characterized ALLase from Escherichia coli BL21. Purified recombinant ALLase exhibited no activity but could be activated when preincubating with some metal ions before analyzing its activity, and was in the order: Mn(2+)- ≫ Co(2+)- > Zn(2+)- > Ni(2+)- > Cd(2+)- ~Mg(2+)-activated enzyme; however, activity of ALLase (Mn(2+)-activated form) was also significantly inhibited with 5 mM Co(2+), Zn(2+), and Cd(2+) ions. Activity of Mn(2+)-activated ALLase was increased by adding the reducing agent dithiothreitol (DTT), but was decreased by treating with the sulfhydryl modifying reagent N-ethylmaleimide (NEM). Inhibition of Mn(2+)-activated ALLase by chelator 8-hydroxy-5-quinolinesulfonic acid (8-HQSA), but not EDTA, was pH-dependent. Analysis of purified ALLase by gel filtration chromatography revealed a mixture of monomers, dimers, and tetramers. Substituting the putative metal binding residues His59, His61, Lys146, His186, His242, and Asp315 with Ala completely abolished the activity of ALLase, even preincubating with Mn(2+) ions. On the basis of these results, as well as the pH-activity profile, the reaction mechanism of ALLase is discussed and compared with those of other cyclic amidohydrolases.

Characterization of a single-stranded DNA binding protein from Salmonella enterica serovar Typhimurium LT2

Single-stranded DNA-binding protein (SSB) plays an important role in DNA metabolism, such as DNA replication, repair, and recombination, and is essential for cell survival. We characterized the single-stranded DNA (ssDNA)-binding properties of Salmonella enterica serovar Typhimurium LT2 SSB (StSSB) by using fluorescence quenching measurements and electrophoretic mobility shift analysis (EMSA). Analysis of purified StSSB by gel filtration chromatography showed a stable tetramer in solution. In fluorescence titrations, StSSB bound to 21-38 nucleotides (nt) per tetramer depending on the salt concentration. Using EMSA, we characterized the stoichiometry of StSSB complexed with a series of ssDNA homopolymers, and the size of the binding site was determined to be 22 ± 1 nt. Furthermore, EMSA results indicated that the dissociation constants of StSSB for the first tetramer were less than that for the second tetramer. On the basis of these biophysical analyses, the ssDNA binding-mode of StSSB is expected to be noncooperative.