From isotopically labeled DNA to fluorescently labeled dynamic pili: building a mechanistic model of DNA transport to the cytoplasmic membrane

SUMMARYNatural competence, the physiological state wherein bacteria produce proteins that mediate extracellular DNA transport into the cytosol and the subsequent recombination of DNA into the genome, is conserved across the bacterial domain. DNA must successfully translocate across formidable permeability barriers during import, including the cell membrane(s) and the cell wall, that are normally impermeable to large DNA polymers. This review will examine the mechanisms underlying DNA transport from the extracellular space to the cytoplasmic membrane. First, the challenges inherent to DNA movement through the cell periphery will be discussed to provide context for DNA transport during natural competence. The following sections will trace the development of a comprehensive model for DNA translocation to the cytoplasmic membrane, highlighting the crucial studies performed over the last century that have contributed to building contemporary DNA import models. Finally, this review will conclude by reflecting on what is still unknown about the process and the possible solutions to overcome these limitations.

Visualizing dynamic competence pili and DNA capture throughout the long axis of Bacillus subtilis

The first step in the process of bacterial natural transformation is DNA capture. Although long hypothesized based on genetics and functional experiments, the pilus structure responsible for initial DNA binding had not yet been visualized for Bacillus subtilis. Here, we visualize functional competence pili in Bacillus subtilis using fluorophore-conjugated maleimide labeling in conjunction with epifluorescence microscopy. In strains that produce pilin monomers within tenfold of wild-type levels, the median length of detectable pili is 300 nm. These pili are retractile and associate with DNA. The analysis of pilus distribution at the cell surface reveals that they are predominantly located along the long axis of the cell. The distribution is consistent with localization of proteins associated with subsequent transformation steps, DNA binding, and DNA translocation in the cytosol. These data suggest a distributed model for B. subtilis transformation machinery, in which initial steps of DNA capture occur throughout the long axis of the cell and subsequent steps may also occur away from the cell poles. IMPORTANCE This work provides novel visual evidence for DNA translocation across the cell wall during Bacillus subtilis natural competence, an essential step in the natural transformation process. Our data demonstrate the existence of natural competence-associated retractile pili that can bind exogenous DNA. Furthermore, we show that pilus biogenesis occurs throughout the cell long axis. These data strongly support DNA translocation occurring all along the lateral cell wall during natural competence, wherein pili are produced, bind to free DNA in the extracellular space, and finally retract to pull the bound DNA through the gap in the cell wall created during pilus biogenesis.

Visualizing dynamic competence pili and DNA capture throughout the long axis of Bacillus subtilis

The first step in the process of bacterial natural transformation is DNA capture. Although long-hypothesized based on genetics and functional experiments, the pilus structure responsible for initial DNA-binding had not yet been visualized for Bacillus subtilis. Here, we visualize functional competence pili in Bacillus subtilis using fluorophore-conjugated maleimide labeling in conjunction with epifluorescence microscopy. In strains that produce pilin monomers within ten-fold of wild type levels, the median length of detectable pili is 300nm. These pili are retractile and associate with DNA. Analysis of pilus distribution at the cell surface reveals that they are predominantly located along the long axis of the cell. The distribution is consistent with localization of proteins associated with subsequent transformation steps, DNA-binding and DNA translocation in the cytosol. These data suggest a distributed model for B. subtilis transformation machinery, in which initial steps of DNA capture occur throughout the long axis of the cell and subsequent steps may also occur away from the cell poles.

Programming bacteria for multiplexed DNA detection

DNA is a universal and programmable signal of living organisms. Here we develop cell-based DNA sensors by engineering the naturally competent bacterium Bacillus subtilis (B. subtilis) to detect specific DNA sequences in the environment. The DNA sensor strains can identify diverse bacterial species including major human pathogens with high specificity. Multiplexed detection of genomic DNA from different species in complex samples can be achieved by coupling the sensing mechanism to orthogonal fluorescent reporters. We also demonstrate that the DNA sensors can detect the presence of species in the complex samples without requiring DNA extraction. The modularity of the living cell-based DNA-sensing mechanism and simple detection procedure could enable programmable DNA sensing for a wide range of applications.

Efficient plasmid transfer via natural competence in a microbial co-culture

The molecular and ecological factors shaping horizontal gene transfer (HGT) via natural transformation in microbial communities are largely unknown, which is critical for understanding the emergence of antibiotic-resistant pathogens. We investigate key factors shaping HGT in a microbial co-culture by quantifying extracellular DNA release, species growth, and HGT efficiency over time. In the co-culture, plasmid release and HGT efficiency are significantly enhanced than in the respective monocultures. The donor is a key determinant of HGT efficiency as plasmids induce the SOS response, enter a multimerized state, and are released in high concentrations, enabling efficient HGT. However, HGT is reduced in response to high donor lysis rates. HGT is independent of the donor viability state as both live and dead cells transfer the plasmid with high efficiency. In sum, plasmid HGT via natural transformation depends on the interplay of plasmid properties, donor stress responses and lysis rates, and interspecies interactions.

An electric alarm clock for spores

Inactive spores integrate stimuli over time through stored electrochemical potential.

Designing efficient genetic code expansion in Bacillus subtilis to gain biological insights

Bacillus subtilis is a model gram-positive bacterium, commonly used to explore questions across bacterial cell biology and for industrial uses. To enable greater understanding and control of proteins in B. subtilis, here we report broad and efficient genetic code expansion in B. subtilis by incorporating 20 distinct non-standard amino acids within proteins using 3 different families of genetic code expansion systems and two choices of codons. We use these systems to achieve click-labelling, photo-crosslinking, and translational titration. These tools allow us to demonstrate differences between E. coli and B. subtilis stop codon suppression, validate a predicted protein-protein binding interface, and begin to interrogate properties underlying bacterial cytokinesis by precisely modulating cell division dynamics in vivo. We expect that the establishment of this simple and easily accessible chemical biology system in B. subtilis will help uncover an abundance of biological insights and aid genetic code expansion in other organisms.

Pervasive prophage recombination occurs during evolution of spore-forming Bacilli

Phages are the main source of within-species bacterial diversity and drivers of horizontal gene transfer, but we know little about the mechanisms that drive genetic diversity of these mobile genetic elements (MGEs). Recently, we showed that a sporulation selection regime promotes evolutionary changes within SPβ prophage of Bacillus subtilis, leading to direct antagonistic interactions within the population. Herein, we reveal that under a sporulation selection regime, SPβ recombines with low copy number phi3Ts phage DNA present within the B. subtilis population. Recombination results in a new prophage occupying a different integration site, as well as the spontaneous release of virulent phage hybrids. Analysis of Bacillus sp. strains suggests that SPβ and phi3T belong to a distinct cluster of unusually large phages inserted into sporulation-related genes that are equipped with a spore-related genetic arsenal. Comparison of Bacillus sp. genomes indicates that similar diversification of SPβ-like phages takes place in nature. Our work is a stepping stone toward empirical studies on phage evolution, and understanding the eco-evolutionary relationships between bacteria and their phages. By capturing the first steps of new phage evolution, we reveal striking relationship between survival strategy of bacteria and evolution of their phages.

Quantitative Transformation Efficiency Assay for Bacillus subtilis

Bacillus subtilis (B. subtilis) is a model Gram-positive organism used to study a variety of physiological states and stress responses, one of which is the development of competence. Competence refers to the physiological state of a cell which allows it to be transformed naturally. Through induction of competence, the efficiency of natural transformation can be quantified by plating colony forming units (CFU) and transforming units (TFU). Here we describe a protocol for quantifying relative transformability using B. subtilis.

Image-based modeling of acute myocardial ischemia using experimentally derived ischemic zone source representations

BACKGROUND

Computational models of myocardial ischemia often use oversimplified ischemic source representations to simulate epicardial potentials. The purpose of this study was to explore the influence of biophysically justified, subject-specific ischemic zone representations on epicardial potentials.

METHODS

We developed and implemented an image-based simulation pipeline, using intramural recordings from a canine experimental model to define subject-specific ischemic regions within the heart. Static epicardial potential distributions, reflective of ST segment deviations, were simulated and validated against measured epicardial recordings.

RESULTS

Simulated epicardial potential distributions showed strong statistical correlation and visual agreement with measured epicardial potentials. Additionally, we identified and described in what way border zone parameters influence epicardial potential distributions during the ST segment.

CONCLUSION

From image-based simulations of myocardial ischemia, we generated subject-specific ischemic sources that accurately replicated epicardial potential distributions. Such models are essential in understanding the underlying mechanisms of the bioelectric fields that arise during ischemia and are the basis for more sophisticated simulations of body surface ECGs.

A Conserved Metal Binding Motif in the Bacillus subtilis Competence Protein ComFA Enhances Transformation

Genetic competence is a process in which cells are able to take up DNA from their environment, resulting in horizontal gene transfer, a major mechanism for generating diversity in bacteria. Many bacteria carry homologs of the central DNA uptake machinery that has been well characterized in Bacillus subtilis It has been postulated that the B. subtilis competence helicase ComFA belongs to the DEAD box family of helicases/translocases. Here, we made a series of mutants to analyze conserved amino acid motifs in several regions of B. subtilis ComFA. First, we confirmed that ComFA activity requires amino acid residues conserved among the DEAD box helicases, and second, we show that a zinc finger-like motif consisting of four cysteines is required for efficient transformation. Each cysteine in the motif is important, and mutation of at least two of the cysteines dramatically reduces transformation efficiency. Further, combining multiple cysteine mutations with the helicase mutations shows an additive phenotype. Our results suggest that the helicase and metal binding functions are two distinct activities important for ComFA function during transformation.IMPORTANCE ComFA is a highly conserved protein that has a role in DNA uptake during natural competence, a mechanism for horizontal gene transfer observed in many bacteria. Investigation of the details of the DNA uptake mechanism is important for understanding the ways in which bacteria gain new traits from their environment, such as drug resistance. To dissect the role of ComFA in the DNA uptake machinery, we introduced point mutations into several motifs in the protein sequence. We demonstrate that several amino acid motifs conserved among ComFA proteins are important for efficient transformation. This report is the first to demonstrate the functional requirement of an amino-terminal cysteine motif in ComFA.

Missense Mutations Allow a Sequence-Blind Mutant of SpoIIIE to Successfully Translocate Chromosomes during Sporulation

SpoIIIE directionally pumps DNA across membranes during Bacillus subtilis sporulation and vegetative growth. The sequence-reading domain (γ domain) is required for directional DNA transport, and its deletion severely impairs sporulation. We selected suppressors of the spoIIIEΔγ sporulation defect. Unexpectedly, many suppressors were intragenic missense mutants, and some restore sporulation to near-wild-type levels. The mutant proteins are likely not more abundant, faster at translocating DNA, or sequence-sensitive, and rescue does not involve the SpoIIIE homolog SftA. Some mutants behave differently when co-expressed with spoIIIEΔγ, consistent with the idea that some, but not all, variants may form mixed oligomers. In full-length spoIIIE, these mutations do not affect sporulation, and yet the corresponding residues are rarely found in other SpoIIIE/FtsK family members. The suppressors do not rescue chromosome translocation defects during vegetative growth, indicating that the role of the γ domain cannot be fully replaced by these mutations. We present two models consistent with our findings: that the suppressors commit to transport in one arbitrarily-determined direction or delay spore development. It is surprising that missense mutations somehow rescue loss of an entire domain with a complex function, and this raises new questions about the mechanism by which SpoIIIE pumps DNA and the roles SpoIIIE plays in vivo.

Do the same traffic rules apply? Directional chromosome segregation by SpoIIIE and FtsK

Over a decade of studies have tackled the question of how FtsK/SpoIIIE translocases establish and maintain directional DNA translocation during chromosome segregation in bacteria. FtsK/SpoIIIE translocases move DNA in a highly processive, directional manner, where directionality is facilitated by sequences on the substrate DNA molecules that are being transported. In recent years, structural, biochemical, single-molecule and high-resolution microscopic studies have provided new insight into the mechanistic details of directional DNA segregation. Out of this body of work, a series of models have emerged and, ultimately, yielded two seemingly opposing models: the loading model and the target search model. We review these recent mechanistic insights into directional DNA movement and discuss the data that may serve to unite these suggested models, as well as propose future directions that may ultimately solve the debate.

Linked domain architectures allow for specialization of function in the FtsK/SpoIIIE ATPases of ESX secretion systems

Among protein secretion systems, there are specialized ATPases that serve different functions such as substrate recognition, substrate unfolding, and assembly of the secretory machinery. ESX (early secretory antigen target 6 kDa secretion) protein secretion systems require FtsK/SpoIIIE family ATPases but the specific function of these ATPases is poorly understood. The ATPases of ESX secretion systems have a unique domain architecture among proteins of the FtsK/SpoIIIE family. All well-studied FtsK family ATPases to date have one ATPase domain and oligomerize to form a functional molecular machine, most commonly a hexameric ring. In contrast, the ESX ATPases have three ATPase domains, encoded either by a single gene or by two operonic genes. It is currently unknown which of the ATPase domains is catalytically functional and whether each domain plays the same or a different function. Here we focus on the ATPases of two ESX systems, the ESX-1 system of Mycobacterium tuberculosis and the yuk system of Bacillus subtilis. We show that ATP hydrolysis by the ESX ATPase is required for secretion, suggesting that this enzyme at least partly fuels protein translocation. We further show that individual ATPase domains play distinct roles in substrate translocation and complex formation. Comparing the single-chain and split ESX ATPases, we reveal differences in the requirements of these unique secretory ATPases.

The ESX system in Bacillus subtilis mediates protein secretion

Esat-6 protein secretion systems (ESX or Ess) are required for the virulence of several human pathogens, most notably Mycobacterium tuberculosis and Staphylococcus aureus. These secretion systems are defined by a conserved FtsK/SpoIIIE family ATPase and one or more WXG100 family secreted substrates. Gene clusters coding for ESX systems have been identified amongst many organisms including the highly tractable model system, Bacillus subtilis. In this study, we demonstrate that the B. subtilis yuk/yue locus codes for a nonessential ESX secretion system. We develop a functional secretion assay to demonstrate that each of the locus gene products is specifically required for secretion of the WXG100 virulence factor homolog, YukE. We then employ an unbiased approach to search for additional secreted substrates. By quantitative profiling of culture supernatants, we find that YukE may be the sole substrate that depends on the FtsK/SpoIIIE family ATPase for secretion. We discuss potential functional implications for secretion of a unique substrate.

SpoIIIE protein achieves directional DNA translocation through allosteric regulation of ATPase activity by an accessory domain

Bacterial chromosome segregation utilizes highly conserved directional translocases of the SpoIIIE/FtsK family. These proteins employ an accessory DNA-binding domain (γ) to dictate directionality of DNA transport. It remains unclear how the interaction of γ with specific recognition sequences coordinates directional DNA translocation. We demonstrate that the γ domain of SpoIIIE inhibits ATPase activity of the motor domain in the absence of DNA but stimulates ATPase activity through sequence-specific DNA recognition. Furthermore, we observe that communication between γ subunits is necessary for both regulatory roles. Consistent with these findings, the γ domain is necessary for robust DNA transport along the length of the chromosome in vivo. Together, our data reveal that directional activation involves allosteric regulation of ATP turnover through coordinated action of γ domains. Thus, we propose a coordinated stimulation model in which γ-γ communication is required to translate DNA sequence information from each γ to its respective motor domain.

FisB mediates membrane fission during sporulation in Bacillus subtilis

How bacteria catalyze membrane fission during growth and differentiation is an outstanding question in prokaryotic cell biology. Here, we describe a protein (FisB, for fission protein B) that mediates membrane fission during the morphological process of spore formation in Bacillus subtilis. Sporulating cells divide asymmetrically, generating a large mother cell and smaller forespore. After division, the mother cell membranes migrate around the forespore in a phagocytic-like process called engulfment. Membrane fission releases the forespore into the mother cell cytoplasm. Cells lacking FisB are severely and specifically impaired in the fission reaction. Moreover, GFP-FisB forms dynamic foci that become immobilized at the site of fission. Purified FisB catalyzes lipid mixing in vitro and is only required in one of the fusing membranes, suggesting that FisB-lipid interactions drive membrane remodeling. Consistent with this idea, the extracytoplasmic domain of FisB binds with remarkable specificity to cardiolipin, a lipid enriched in the engulfing membranes and regions of negative curvature. We propose that membrane topology at the final stage of engulfment and FisB-cardiolipin interactions ensure that the mother cell membranes are severed at the right time and place. The unique properties of FisB set it apart from the known fission machineries in eukaryotes, suggesting that it represents a new class of fission proteins.

SpoIIIE strips proteins off the DNA during chromosome translocation

The FtsK/SpoIIIE family of DNA transporters are responsible for translocating missegregated chromosomes after the completion of cell division. An extreme example of this post-cytokinetic DNA segregation occurs during spore formation in the bacterium Bacillus subtilis, where SpoIIIE pumps three-quarters of the chromosome (>3 megabases) into one of the two daughter cells. Here, we investigate the fate of the proteins associated with the translocated DNA. Taking advantage of several unique features of Bacillus sporulation, we demonstrate that RNA polymerase, transcription factors, and chromosome remodeling proteins are stripped off the DNA during translocation of the chromosome into the forespore compartment. Furthermore, we show that in vitro the soluble ATPase domain of SpoIIIE can displace RNA polymerase bound to DNA, suggesting that SpoIIIE alone is capable of this wire-stripping activity. Our data suggest that the bulk of the forespore chromosome is translocated naked into the forespore compartment. We propose that the translocation-stripping activity of SpoIIIE plays a key role in reprogramming developmental gene expression in the forespore.

The ATPase SpoIIIE transports DNA across fused septal membranes during sporulation in Bacillus subtilis

The FtsK/SpoIIIE family of ATP-dependent DNA transporters mediates proper chromosome segregation in dividing bacteria. In sporulating Bacillus subtilis cells, SpoIIIE translocates much of the circular chromosome from the mother cell into the forespore, but the molecular mechanism remains unclear. Using a new assay to monitor DNA transport, we demonstrate that the two arms of the chromosome are simultaneously pumped into the forespore. Up to 70 molecules of SpoIIIE are recruited to the site of DNA translocation and assemble into complexes that could contain 12 subunits. The fusion of the septal membranes during cytokinesis precedes DNA translocation and does not require SpoIIIE, as suggested by analysis of lipid dynamics, serial thin-section electron microscopy, and cell separation by protoplasting. These data support a model for DNA transport in which the transmembrane segments of FtsK/SpoIIIE form linked DNA-conducting channels across the two lipid bilayers of the septum.

Remodeling protein complexes: insights from the AAA+ unfoldase ClpX and Mu transposase

Multiprotein complexes in the cell are dynamic entities that are constantly undergoing changes in subunit composition and conformation to carry out their functions. The protein-DNA complex that promotes recombination of the bacteriophage Mu is a prime example of a complex that must undergo specific changes to carry out its function. The Clp/Hsp100 family of AAA+ ATPases plays a critical role in mediating such changes. The Clp/Hsp100 unfolding enzymes have been extensively studied for the roles they play in protein degradation. However, degradation is not the only fate for proteins that come in contact with the ATP-dependent unfolding enzymes. The Clp/Hsp100 enzymes induce structural changes in their substrates. These structural changes, which we refer to as "remodeling", ultimately change the biological activity of the substrate. These biological changes include activation, inactivation (not associated with degradation), and relocation within the cell. Analysis of the interaction between Escherichia coli ClpX unfoldase and the Mu recombination complex, has provided molecular insight into the mechanisms of protein remodeling. We discuss the key mechanistic features of the remodeling reactions promoted by ClpX and possible implications of these findings for other biological reactions.

Sculpting the proteome with AAA(+) proteases and disassembly machines

Machines of protein destruction-including energy-dependent proteases and disassembly chaperones of the AAA(+) ATPase family-function in all kingdoms of life to sculpt the cellular proteome, ensuring that unnecessary and dangerous proteins are eliminated and biological responses to environmental change are rapidly and properly regulated. Exciting progress has been made in understanding how AAA(+) machines recognize specific proteins as targets and then carry out ATP-dependent dismantling of the tertiary and/or quaternary structure of these molecules during the processes of protein degradation and the disassembly of macromolecular complexes.

Mu transpososome architecture ensures that unfolding by ClpX or proteolysis by ClpXP remodels but does not destroy the complex

The Clp/Hsp100 ATPases are protein unfoldases that both alter protein conformation and target proteins for degradation. An unresolved question has been how such seemingly destructive enzymes can "remodel" some protein substrates rather than destroy them. Here, we investigate the products of ClpX-mediated remodeling of a hyper-stable protein-DNA complex, the Mu transpososome. We find that although an oligomeric complex is maintained, release of some subunits accompanies ClpX action. Replacement of transposase's endogenous ClpX-recognition sequence with an exogenous signal reveals that the mechanism of remodeling is independent of both the recognition signal and the identity of the unfoldase. Finally, examination of the transposase-DNA contacts reveals only a localized region that is altered during remodeling. These results provide a framework for protein remodeling, wherein the physical attributes of a complex can limit the unfolding activity of its remodeler.

ClpX-mediated remodeling of mu transpososomes: selective unfolding of subunits destabilizes the entire complex

E. coli ClpX, a member of the Clp/Hsp100 family of ATPases, remodels multicomponent complexes and facilitates ATP-dependent degradation. Here, we analyze the mechanism by which ClpX destabilizes the exceedingly stable Mu transpososome, a natural substrate for remodeling rather than degradation. We find that ClpX has the capacity to globally unfold transposase monomers, the building blocks of the transpososome. A biochemical probe for protein unfolding reveals that ClpX also unfolds MuA subunits during remodeling reactions, but that not all subunits have their structure extensively modified. In fact, direct recognition and unfolding of a single transposase subunit are sufficient for ClpX to destabilize the entire transpososome. Thus, the ability of ClpX to unfold proteins is sufficient to explain its role in both complex destabilization and ATP-dependent proteolysis.

Genetic interactions between the Escherichia coli umuDC gene products and the beta processivity clamp of the replicative DNA polymerase

The Escherichia coli umuDC gene products encode DNA polymerase V, which participates in both translesion DNA synthesis (TLS) and a DNA damage checkpoint control. These two temporally distinct roles of the umuDC gene products are regulated by RecA-single-stranded DNA-facilitated self-cleavage of UmuD (which participates in the checkpoint control) to yield UmuD' (which enables TLS). In addition, even modest overexpression of the umuDC gene products leads to a cold-sensitive growth phenotype, apparently due to the inappropriate expression of the DNA damage checkpoint control activity of UmuD(2)C. We have previously reported that overexpression of the epsilon proofreading subunit of DNA polymerase III suppresses umuDC-mediated cold sensitivity, suggesting that interaction of epsilon with UmuD(2)C is important for the DNA damage checkpoint control function of the umuDC gene products. Here, we report that overexpression of the beta processivity clamp of the E. coli replicative DNA polymerase (encoded by the dnaN gene) not only exacerbates the cold sensitivity conferred by elevated levels of the umuDC gene products but, in addition, confers a severe cold-sensitive phenotype upon a strain expressing moderately elevated levels of the umuD'C gene products. Such a strain is not otherwise normally cold sensitive. To identify mutant beta proteins possibly deficient for physical interactions with the umuDC gene products, we selected for novel dnaN alleles unable to confer a cold-sensitive growth phenotype upon a umuD'C-overexpressing strain. In all, we identified 75 dnaN alleles, 62 of which either reduced the expression of beta or prematurely truncated its synthesis, while the remaining alleles defined eight unique missense mutations of dnaN. Each of the dnaN missense mutations retained at least a partial ability to function in chromosomal DNA replication in vivo. In addition, these eight dnaN alleles were also unable to exacerbate the cold sensitivity conferred by modestly elevated levels of the umuDC gene products, suggesting that the interactions between UmuD' and beta are a subset of those between UmuD and beta. Taken together, these findings suggest that interaction of beta with UmuD(2)C is important for the DNA damage checkpoint function of the umuDC gene products. Four possible models for how interactions of UmuD(2)C with the epsilon and the beta subunits of DNA polymerase III might help to regulate DNA replication in response to DNA damage are discussed.

Dynamics of substrate denaturation and translocation by the ClpXP degradation machine

ClpXP is a protein machine composed of the ClpX ATPase, a member of the Clp/Hsp100 family of remodeling enzymes, and the ClpP peptidase. Here, ClpX and ClpXP are shown to catalyze denaturation of GFP modified with an ssrA degradation tag. ClpX translocates this denatured protein into the proteolytic chamber of ClpP and, when proteolysis is blocked, also catalyzes release of denatured GFP-ssrA from ClpP in a reaction that requires ATP and additional substrate. Kinetic experiments reveal that multiple reaction steps require collaboration between ClpX and ClpP and that denaturation is the rate-determining step in degradation. These insights into the mechanism of ClpXP explain how it executes efficient degradation in a manner that is highly specific for tagged proteins, irrespective of their intrinsic stabilities.