E. coli SSB tetramer binds the first and second molecules of (dT)(35) with heat capacities of opposite sign

Engineering a reagentless biosensor for single-stranded DNA to measure real-time helicase activity in Bacillus

Single-stranded DNA-binding protein (SSB) is a well characterized ubiquitous and essential bacterial protein involved in almost all aspects of DNA metabolism. Using the Bacillus subtilis SSB we have generated a reagentless SSB biosensor that can be used as a helicase probe in B. subtilis and closely related gram positive bacteria. We have demonstrated the utility of the probe in a DNA unwinding reaction using a helicase from Bacillus and for the first time, characterized the B. subtilis SSB's DNA binding mode switching and stoichiometry. The importance of SSB in DNA metabolism is not limited to simply binding and protecting ssDNA during DNA replication, as previously thought. It interacts with an array of partner proteins to coordinate many different aspects of DNA metabolism. In most cases its interactions with partner proteins is species-specific and for this reason, knowing how to produce and use cognate reagentless SSB biosensors in different bacteria is critical. Here we explain how to produce a B. subtilis SSB probe that exhibits 9-fold fluorescence increase upon binding to single stranded DNA and can be used in all related gram positive firmicutes which employ drastically different DNA replication and repair systems than the widely studied Escherichia coli. The materials to produce the B. subtilis SSB probe are commercially available, so the methodology described here is widely available unlike previously published methods for the E. coli SSB.

SSB binding to single-stranded DNA probed using solid-state nanopore sensors

Single-stranded DNA (ssDNA) binding protein plays an important role in the DNA replication process in a wide range of organisms. It binds to ssDNA to prevent premature reannealing and to protect it from degradation. Current understanding of SSB/ssDNA interaction points to a complex mechanism, including SSB motion along the DNA strand. We report on the first characterization of this interaction at the single-molecule level using solid-state nanopore sensors, namely without any labeling or surface immobilization. Our results show that the presence of SSB on the ssDNA can control the speed of nanopore translocation, presumably due to strong interactions between SSB and the nanopore surface. This enables nanopore-based detection of ssDNA fragments as short as 37 nt, which is normally very difficult with solid-state nanopore sensors, due to constraints in noise and bandwidth. Notably, this fragment is considerably shorter than the 65 nt binding motif, typically required for SSB binding at high salt concentrations. The nonspecificity of SSB binding to ssDNA further suggests that this approach could be used for fragment sizing of short ssDNA.

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.

Differential temperature-dependent multimeric assemblies of replication and repair polymerases on DNA increase processivity

Differentiation of binding accurate DNA replication polymerases over error prone DNA lesion bypass polymerases is essential for the proper maintenance of the genome. The hyperthermophilic archaeal organism Sulfolobus solfataricus (Sso) contains both a B-family replication (Dpo1) and a Y-family repair (Dpo4) polymerase and serves as a model system for understanding molecular mechanisms and assemblies for DNA replication and repair protein complexes. Protein cross-linking, isothermal titration calorimetry, and analytical ultracentrifugation have confirmed a previously unrecognized dimeric Dpo4 complex bound to DNA. Binding discrimination between these polymerases on model DNA templates is complicated by the fact that multiple oligomeric species are influenced by concentration and temperature. Temperature-dependent fluorescence anisotropy equilibrium binding experiments were used to separate discrete binding events for the formation of trimeric Dpo1 and dimeric Dpo4 complexes on DNA. The associated equilibria are found to be temperature-dependent, generally leading to improved binding at higher temperatures for both polymerases. At high temperatures, DNA binding of Dpo1 monomer is favored over binding of Dpo4 monomer, but binding of Dpo1 trimer is even more strongly favored over binding of Dpo4 dimer, thus providing thermodynamic selection. Greater processivities of nucleotide incorporation for trimeric Dpo1 and dimeric Dpo4 are also observed at higher temperatures, providing biochemical validation for the influence of tightly bound oligomeric polymerases. These results separate, quantify, and confirm individual and sequential processes leading to the formation of oligomeric Dpo1 and Dpo4 assemblies on DNA and provide for a concentration- and temperature-dependent discrimination of binding undamaged DNA templates at physiological temperatures.

Plasmodium falciparum SSB tetramer binds single-stranded DNA only in a fully wrapped mode

The tetrameric Escherichia coli single-stranded DNA (ssDNA) binding protein (Ec-SSB) functions in DNA metabolism by binding to ssDNA and interacting directly with numerous DNA repair and replication proteins. Ec-SSB tetramers can bind ssDNA in multiple DNA binding modes that differ in the extent of ssDNA wrapping. Here, we show that the structurally similar SSB protein from the malarial parasite Plasmodium falciparum (Pf-SSB) also binds tightly to ssDNA but does not display the same number of ssDNA binding modes as Ec-SSB, binding ssDNA exclusively in fully wrapped complexes with site sizes of 52-65 nt/tetramer. Pf-SSB does not transition to the more cooperative (SSB)(35) DNA binding mode observed for Ec-SSB. Consistent with this, Pf-SSB tetramers also do not display the dramatic intra-tetramer negative cooperativity for binding of a second (dT)(35) molecule that is evident in Ec-SSB. These findings highlight variations in the DNA binding properties of these two highly conserved homotetrameric SSB proteins, and these differences might be tailored to suit their specific functions in the cell.

SSB binding to ssDNA using isothermal titration calorimetry

Isothermal titration calorimetry (ITC) is a powerful method for studying protein-DNA interactions in solution. As long as binding is accompanied by an appreciable enthalpy change, ITC studies can yield quantitative information on stoichiometries, binding energetics (affinity, binding enthalpy and entropy) and potential site-site interactions (cooperativity). This can provide a full thermodynamic description of an interacting system which is necessary to understand the stability and specificity of protein-DNA interactions and to correlate the activities or functions of different species. Here we describe procedures to perform and analyze ITC studies using as examples, the E. coli SSB (homotetramer with 4 OB-folds) and D. radiodurans SSB (homodimer with 4 OB-folds). For oligomeric protein systems such as these, we emphasize the need to be aware of the likelihood that solution conditions will influence not only the affinity and enthalpy of binding but also the mode by which the SSB oligomer binds ssDNA.

Introduction: twenty five years of the Gibbs Conference on Biothermodynamics

In 2011, the Gibbs Conference on Biothermodynamics will celebrate its 25th anniversary. Since the inaugural meeting in 1987, it has brought together laboratories that lived, breathed and argued about the molecular logic of macromolecular machines. The participants have a deep commitment to understanding the nature of physico-chemical forces that govern regulation of biological systems, and share a passion for applying linkage theory. The collective goal is to understand how ligand binding, subunit assembly and conformational change drive what we observe as physiological processes such as regulated transport, enzyme cascades, gene regulation, membrane permeability, viral infection, intracellular trafficking and folding of macromolecules. In this special issue, articles by former organizers of the Gibbs Conference showcase the current breadth and depth of the field of biothermodynamics, and how thoroughly it is integrated with the study of macromolecular structures, computational modeling and physiological studies of human health and disease.