Structure and DNA-binding properties of the cytolysin regulator CylR2 from Enterococcus faecalis

An Overview of the Factors Involved in Biofilm Production by the Enterococcus Genus

Enterococcus species are known for their ability to form biofilms, which contributes to their survival in extreme environments and involvement in persistent bacterial infections, especially in the case of multi-drug-resistant strains. This review aims to provide a comprehensive understanding of the mechanisms underlying biofilm formation in clinically important species such as Enterococcus faecalis and the less studied but increasingly multi-drug-resistant Enterococcus faecium, and explores potential strategies for their eradication. Biofilm formation in Enterococcus involves a complex interplay of genes and virulence factors, including gelatinase, cytolysin, Secreted antigen A, pili, microbial surface components that recognize adhesive matrix molecules (MSCRAMMs), and DNA release. Quorum sensing, a process of intercellular communication, mediated by peptide pheromones such as Cob, Ccf, and Cpd, plays a crucial role in coordinating biofilm development by targeting gene expression and regulation. Additionally, the regulation of extracellular DNA (eDNA) release has emerged as a fundamental component in biofilm formation. In E. faecalis, the autolysin N-acetylglucosaminidase and proteases such as gelatinase and serin protease are key players in this process, influencing biofilm development and virulence. Targeting eDNA may offer a promising avenue for intervention in biofilm-producing E. faecalis infections. Overall, gaining insights into the intricate mechanisms of biofilm formation in Enterococcus may provide directions for anti-biofilm therapeutic research, with the purpose of reducing the burden of Enterococcus-associated infections.

Clostridium ramosum Bacteremia in an Immunocompetent Patient with SARS-CoV-2 Infection: A Case Report

We report a case of Clostridium ramosum bacteremia in a 73-year-old patient with SARS-CoV-2 infection and right lower abdominal tenderness in China. The microbiological features and genomic epidemiological characteristics of C. ramosum worldwide were investigated to identify the possible sources of infection. Whole-genome sequencing of C. ramosum WD-I2 was performed using an Illumina NovaSeq 6000 platform. Phylogenetic analysis of C. ramosum WD-I2 and other publicly available C. ramosum isolates was performed and visualized using the interactive Tree of Life (iTOL) web server. The resistome of C. ramosum WD-I2 consists of two antimicrobial resistance genes (tetM and ermB), which explains the antimicrobial resistance trait to tetracycline and macrolides. Phylogenetic analysis showed that the strain closest to our isolated strain WD-I2 was SUG1069, recovered from a pig feces sample from Canada, which differed by 589 SNPs. To our knowledge, this is the first report of C. ramosum bacteremia in China. Our findings highlight the potential risk of invasive C. ramosum infections during the COVID-19 pandemic.

Hidden agenda of Enterococcus faecalis lifestyle transition: planktonic to sessile state

Enterococcus faecalis, a human gastrointestinal tract commensal, is known to cause nosocomial infections. Interestingly, the pathogen's host colonization and persistent infections are possibly linked to its lifestyle changes from planktonic to sessile state. Also, the multidrug resistance and survival fitness acquired in the sessile stage of E. faecalis has challenged treatment regimes. This situation exists because of the critical role played by several root genes and their molecular branches, which are part of quorum sensing, aggregation substance, surface adhesions, stress-related response and sex pheromones in the sessile state. It is therefore imperative to decode the hidden agenda of E. faecalis and understand the significant factors influencing biofilm formation. This would, in turn, augment the development of novel strategies to tackle E. faecalis infections.

Anti-Biofilm and Antibacterial Activities of Cycas media R. Br Secondary Metabolites: In Silico, In Vitro, and In Vivo Approaches

Enterococcus species possess many virulence factors that have an essential role in exacerbating the infections caused by them. The current study aimed to evaluate the effect of the secondary metabolites ginkgetin (GINK) and sotetsuflavone (SOTE), isolated from Cycas media R. Br dichloromethane fraction, on Enterococcus faecalis (E. faecalis) isolates for the first time. The antibacterial and antivirulence activities of the isolated compounds were investigated using docking studies and in vitro by determination of the minimum inhibitory concentrations (MICs). Additionally, flow cytometry and scanning electron microscope (SEM) were utilized to assess the effect of SOTE on the tested bacteria. Moreover, crystal violet assay and qRT-PCR were used to test the effect of SOTE on the biofilm-forming ability of E. faecalis isolates. In addition, a systemic infection model was utilized in vivo to investigate the antibacterial activity of SOTE. We found that both GINK and SOTE showed a good affinity for the five proteins enrolled in the virulence of E. faecalis, with SOTE being the highest, suggesting the possible mechanisms for the antivirulence activity of both ligands. In addition, SOTE exhibited a higher antibacterial activity than GINK, as the values of the MICs of SOTE were lower than those of GINK. Thus, we performed the in vitro and in vivo assays on SOTE. However, they did not exhibit any significant variations (p > 0.05) in the membrane depolarization of E. faecalis isolates. Moreover, as evaluated by SEM, SOTE caused distortion and deformation in the treated cells. Regarding its impact on the biofilm formation, it inhibited the biofilm-forming ability of the tested isolates, as determined by crystal violet assay and qRT-PCR. The in vivo experiment revealed that SOTE resulted in a reduction of the inflammation of the liver and spleen with an increase in the survival rate. SOTE also improved the liver-function tests and decreased tumor necrosis factor-alpha using immunostaining and the inflammation markers, interleukins (IL-1β and IL-6), using ELISA. Thus, we can conclude that SOTE could be a promising compound that should be investigated in future preclinical and clinical studies.

Crystal structure report of the ImmR transcriptional regulator DNA-binding domain of the Bacillus subtilis ICEBs1 transposon

Bacillus subtilis is a commensal member of the human oral and gut microbiomes, which can become infectious to immunocompromised patients. It possesses a conjugative transposon, ICEBs1, which includes > 20 genes and can be passed by horizontal gene transfer to other bacteria, including pathogenic Bacillus anthracis and Listeria monocytogenes. ICEBs1 is regulated by the ImmR/ImmA tandem, which are a transcriptional repressor that constitutively blocks transcription and a metallopeptidase that acts as anti-repressor and inactivates ImmR by proteolytic cleavage. We here report the production and purification of 127-residue ImmR from ICEBs1 and the crystal structure of its DNA-binding domain. It features a five-helix bundle centred on a helix-turn-helix motif potentially binding the major grove of double-stranded target DNA. ImmR shows structural and mechanistic similarity with the B. subtilis SinR repressor, which is engaged in sporulation inhibition.

The near-symmetry of protein oligomers: NMR-derived structures

The majority of oligomeric proteins form clusters which have rotational or dihedral symmetry. Despite the many advantages of symmetric packing, protein oligomers are only nearly symmetric, and the origin of this phenomenon is still in need to be fully explored. Here we apply near-symmetry analyses by the Continuous Symmetry Measures methodology of protein homomers to their natural state, namely their structures in solution. NMR-derived structural data serves us for that purpose. We find that symmetry deviations of proteins are by far higher in solution, compared to the crystalline state; that much of the symmetry distortion is due to amino acids along the interface between the subunits; that the distortions are mainly due to hydrophilic amino acids; and that distortive oligomerization processes such as the swap-domain mechanism can be identified by the symmetry analysis. Most of the analyses were carried out on distorted C2-symmetry dimers, but C3 and D2 cases were analyzed as well. Our NMR analysis supports the idea that the crystallographic B-factor represents non-classical crystals, in which different conformers pack in the crystal, perhaps from the conformers which the NMR analysis provides.

Crystal structure of an anti-CRISPR protein, AcrIIA1

Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide bacteria with RNA-based adaptive immunity against phage infection. To counteract this defense mechanism, phages evolved anti-CRISPR (Acr) proteins that inactivate the CRISPR-Cas systems. AcrIIA1, encoded by Listeria monocytogenes prophages, is the most prevalent among the Acr proteins targeting type II-A CRISPR-Cas systems and has been used as a marker to identify other Acr proteins. Here, we report the crystal structure of AcrIIA1 and its RNA-binding affinity. AcrIIA1 forms a dimer with a novel two helical-domain architecture. The N-terminal domain of AcrIIA1 exhibits a helix-turn-helix motif similar to transcriptional factors. When overexpressed in Escherichia coli, AcrIIA1 associates with RNAs, suggesting that AcrIIA1 functions via nucleic acid recognition. Taken together, the unique structural and functional features of AcrIIA1 suggest its distinct mode of Acr activity, expanding the diversity of the inhibitory mechanisms employed by Acr proteins.

Enterococcal Genetics

The study of the genetics of enterococci has focused heavily on mobile genetic elements present in these organisms, the complex regulatory circuits used to control their mobility, and the antibiotic resistance genes they frequently carry. Recently, more focus has been placed on the regulation of genes involved in the virulence of the opportunistic pathogenic species Enterococcus faecalis and Enterococcus faecium. Little information is available concerning fundamental aspects of DNA replication, partition, and division; this article begins with a brief overview of what little is known about these issues, primarily by comparison with better-studied model organisms. A variety of transcriptional and posttranscriptional mechanisms of regulation of gene expression are then discussed, including a section on the genetics and regulation of vancomycin resistance in enterococci. The article then provides extensive coverage of the pheromone-responsive conjugation plasmids, including sections on regulation of the pheromone response, the conjugative apparatus, and replication and stable inheritance. The article then focuses on conjugative transposons, now referred to as integrated, conjugative elements, or ICEs, and concludes with several smaller sections covering emerging areas of interest concerning the enterococcal mobilome, including nonpheromone plasmids of particular interest, toxin-antitoxin systems, pathogenicity islands, bacteriophages, and genome defense.

Expression of Genes Involved in Bacteriocin Production and Self-Resistance in Lactobacillus brevis 174A Is Mediated by Two Regulatory Proteins

We have previously shown that the lactic acid bacterium Lactobacillus brevis 174A, isolated from Citrus iyo fruit, produces a bacteriocin designated brevicin 174A, which is comprised of two antibacterial polypeptides (designated brevicins 174A-β and 174A-γ). We have also found a gene cluster, composed of eight open reading frames (ORFs), that contains genes for the biosynthesis of brevicin 174A, self-resistance to its own bacteriocin, and two transcriptional regulatory proteins. Some lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth. Generally, the system consists of a membrane-bound histidine protein kinase (HPK) that senses a specific environmental stimulus and a corresponding response regulator (RR) that mediates the cellular response. We have previously shown that although the HPK- and RR-encoding genes are not found on the brevicin 174A biosynthetic gene cluster in the 174A strain, two putative regulatory genes, designated breD and breG, are in the gene cluster. In the present study, we demonstrate that the expression of brevicin 174A production and self-resistance is positively controlled by two transcriptional regulatory proteins, designated BreD and BreG. BreD is expressed together with BreE as the self-resistance determinant of L. brevis 174A. DNase I footprinting analysis and a promoter assay demonstrated that BreD binds to the breED promoter as a positive autoregulator. The present study also demonstrates that BreG, carrying a transmembrane domain, binds to the common promoter of breB and breC, encoding brevicins 174A-β and 174A-γ, respectively, for positive regulation.IMPORTANCE The problem of the appearance of bacteria that are resistant to practical antibiotics and the increasing demand for safe foods have increased interest in replacing conventional antibiotics with bacteriocin produced by the lactic acid bacteria. This antibacterial substance can inhibit the growth of pathogenic bacteria without side effects on the human body. The bacteriocin that is produced by a Citrus iyo-derived Lactobacillus brevis strain inhibits the growth of pathogenic bacteria such as Listeria monocytogenes, Staphylococcus aureus, and Streptococcus mutans In general, lactic acid bacterial strains have a system to start the production of bacteriocin at an adequate stage of growth, which is called a quorum-sensing system. The system consists of a membrane-bound histidine protein kinase that senses a specific environmental stimulus and a corresponding response regulator that mediates the cellular response. The present study demonstrates that the expression of the genes encoding bacteriocin biosynthesis and the self-resistance determinant is positively controlled by two transcriptional regulatory proteins.

Molecular Mechanism of Quorum-Sensing in Enterococcus faecalis: Its Role in Virulence and Therapeutic Approaches

Quorum-sensing systems control major virulence determinants in Enterococcusfaecalis, which causes nosocomial infections. The E. faecalis quorum-sensing systems include several virulence factors that are regulated by the cytolysin operon, which encodes the cytolysin toxin. In addition, the E. faecalis Fsr regulator system controls the expression of gelatinase, serine protease, and enterocin O16. The cytolysin and Fsr virulence factor systems are linked to enterococcal diseases that affect the health of humans and other host models. Therefore, there is substantial interest in understanding and targeting these regulatory pathways to develop novel therapies for enterococcal infection control. Quorum-sensing inhibitors could be potential therapeutic agents for attenuating the pathogenic effects of E. faecalis. Here, we discuss the regulation of cytolysin, the LuxS system, and the Fsr system, their role in E. faecalis-mediated infections, and possible therapeutic approaches to prevent E. faecalis infection.

Effect of the quorum-sensing luxS gene on biofilm formation by Enterococcus faecalis

Enterococcus faecalis is the species of bacterium most frequently isolated from the root canals of teeth that exhibit chronic apical periodontitis refractory to endodontic treatment. In this study, we evaluated the effect of the S-ribosylhomocysteine lyase (luxS) quorum-sensing gene on E. faecalis biofilm formation by constructing a knockout mutant. The biofilms formed by both E. faecalis and its luxS mutant strain were evaluated using the MTT method. Important parameters that influence biofilm formation, including cell-surface hydrophobicity and the nutrient content of the growth medium, were also studied. Biofilm structures were observed using confocal laser scanning microscopy (CLSM), and expression of biofilm-related genes was investigated using RT-PCR. The results showed that the luxS gene can affect biofilm formation, whereas it does not affect the bacterial growth rate. Deletion of the luxS gene also increased cell-surface hydrophobicity. Biofilm formation was accelerated by the addition of increasing concentrations of glucose. The CLSM images revealed that the luxS mutant strain tends to aggregate into distinct clusters and relatively dense structures, whereas the wild-type strain appears confluent and more evenly distributed. All genes examined were up-regulated in the biofilms formed by the luxS mutant strain. The quorum-sensing luxS gene can affect E. faecalis biofilm formation.

Virulence Plasmids of Nonsporulating Gram-Positive Pathogens

Gram-positive bacteria are leading causes of many types of human infection, including pneumonia, skin and nasopharyngeal infections, as well as urinary tract and surgical wound infections among hospitalized patients. These infections have become particularly problematic because many of the species causing them have become highly resistant to antibiotics. The role of mobile genetic elements, such as plasmids, in the dissemination of antibiotic resistance among Gram-positive bacteria has been well studied; less well understood is the role of mobile elements in the evolution and spread of virulence traits among these pathogens. While these organisms are leading agents of infection, they are also prominent members of the human commensal ecology. It appears that these bacteria are able to take advantage of the intimate association between host and commensal, via virulence traits that exacerbate infection and cause disease. However, evolution into an obligate pathogen has not occurred, presumably because it would lead to rejection of pathogenic organisms from the host ecology. Instead, in organisms that exist as both commensal and pathogen, selection has favored the development of mechanisms for variability. As a result, many virulence traits are localized on mobile genetic elements, such as virulence plasmids and pathogenicity islands. Virulence traits may occur within a minority of isolates of a given species, but these minority populations have nonetheless emerged as a leading problem in infectious disease. This chapter reviews virulence plasmids in nonsporulating Gram-positive bacteria, and examines their contribution to disease pathogenesis.

Transcriptional analysis of the Streptococcus pyogenes salivaricin locus

The sal lantibiotic locus plays an important role in the virulence of Streptococcus pyogenes. Our transcriptional analysis of the sal locus provides new information on the complex regulation of this operon. Transcription of the operon is regulated by a promoter upstream of the operon and by a second internal promoter upstream of the salKRZ genes. Here we identify the location of the internal promoter and provide information on how this promoter is autoregulated by proteins within the locus. We determined by primer extension that the salKR promoter is located within the salY gene and identified several regulatory regions important for expression. The higher activity of the promoter in a salKR deletion strain indicates a role in repression by the SalR response regulator. Further, this promoter had higher activity in a salA deletion strain, implicating corepression or a signaling role for the SalA peptide. Finally, we demonstrate that this promoter can be controlled by host factors. Analysis of transcriptional regulation of this locus provides a better understanding of the function of the sal locus in S. pyogenes pathogenesis.

Structure, function, and biology of the Enterococcus faecalis cytolysin

Enterococcus faecalis is a Gram-positive commensal member of the gut microbiota of a wide range of organisms. With the advent of antibiotic therapy, it has emerged as a multidrug resistant, hospital-acquired pathogen. Highly virulent strains of E. faecalis express a pore-forming exotoxin, called cytolysin, which lyses both bacterial and eukaryotic cells in response to quorum signals. Originally described in the 1930s, the cytolysin is a member of a large class of lanthionine-containing bacteriocins produced by Gram-positive bacteria. While the cytolysin shares some core features with other lantibiotics, it possesses unique characteristics as well. The current understanding of cytolysin biosynthesis, structure/function relationships, and contribution to the biology of E. faecalis are reviewed, and opportunities for using emerging technologies to advance this understanding are discussed.

Structural models of protein-DNA complexes based on interface prediction and docking

Protein-DNA interactions are the physical basis of gene expression and DNA modification. Structural models that reveal these interactions are essential for their understanding. As only a limited number of structures for protein-DNA complexes have been determined by experimental methods, computation methods provide a potential way to fill the need. We have developed the DISPLAR method to predict DNA binding sites on proteins. Predicted binding sites have been used to assist the building of structural models by docking, either by guiding the docking or by selecting near-native candidates from the docked poses. Here we applied the DISPLAR method to predict the DNA binding sites for 20 DNA-binding proteins, which have had their DNA binding sites characterized by NMR chemical shift perturbation. For two of these proteins, the structures of their complexes with DNA have also been determined. With the help of the DISPLAR predictions, we built structural models for these two complexes. Evaluations of both the DNA binding sites for 20 proteins and the structural models of the two protein-DNA complexes against experimental results demonstrate the significant promise of our model-building approach.

Integrated analysis of the conformation of a protein-linked spin label by crystallography, EPR and NMR spectroscopy

Long-range structural information derived from paramagnetic relaxation enhancement observed in the presence of a paramagnetic nitroxide radical is highly useful for structural characterization of globular, modular and intrinsically disordered proteins, as well as protein-protein and protein-DNA complexes. Here we characterized the conformation of a spin-label attached to the homodimeric protein CylR2 using a combination of X-ray crystallography, electron paramagnetic resonance (EPR) and NMR spectroscopy. Close agreement was found between the conformation of the spin label observed in the crystal structure with interspin distances measured by EPR and signal broadening in NMR spectra, suggesting that the conformation seen in the crystal structure is also preferred in solution. In contrast, conformations of the spin label observed in crystal structures of T4 lysozyme are not in agreement with the paramagnetic relaxation enhancement observed for spin-labeled CylR2 in solution. Our data demonstrate that accurate positioning of the paramagnetic center is essential for high-resolution structure determination.

Direct stimulus perception and transcription activation by a membrane-bound DNA binding protein

Few membrane proteins with a role in transcriptional regulation have been studied, and none are able to perceive their respective stimuli and activate transcription of their regulons without the aid of auxiliary proteins. The bacitracin resistance regulator, BcrR, of Enterococcus faecalis is a membrane-bound DNA binding protein and is required for bacitracin-dependent expression of the bacitracin resistance genes, bcrABD. Here, we show that BcrR interacts directly with Zn2+ bacitracin (Kd = 2-5 micropM), but not metal-free bacitracin. A solution-based DNA binding assay demonstrated that the affinity of BcrR for its target DNA is much higher (Kd = 40 nM) than previously found for transmembrane regulators and is comparable to that of soluble DNA binding proteins. A construct of BcrR that lacked the transmembrane domain was unable to bind to DNA, indicating that membrane localization was important for DNA binding. Bacitracin did not cause a change in the DNaseI footprint of BcrR on the bcrA promoter, but in vitro transcription assays with BcrR proteoliposomes showed bacitracin-dependent activation of transcription. These findings demonstrate that BcrR is a bona fide one-component transmembrane signal transduction system, which perceives an extracellular stimulus (presence of bacitracin) and relays it to an intracellular transcriptional response independent of any auxiliary proteins.

Molecular analysis of BcrR, a membrane-bound bacitracin sensor and DNA-binding protein from Enterococcus faecalis

BcrR has been identified as a novel regulatory protein of high level bacitracin resistance encoded by the bcrABD operon in Enterococcus faecalis. The N-terminal domain of BcrR has similarity to the helix-turn-helix motif of DNA-binding proteins, and topological modeling predicts that the C-terminal domain contains four transmembrane alpha-helices. These data have led to the hypothesis that BcrR functions as both a membrane-bound sensor and transducer of bacitracin availability to regulate bcrABD expression. To characterize the bcrABD promoter and identify the promoter elements to which BcrR binds, a series of bcrA-lacZ fusions were constructed. A 69-bp region was identified that was essential for bacitracin-dependent bcrA-lacZ expression. Mutations that targeted this region were used to identify two inverted repeat sequences, each with the sequence 5'-GACA(N)(7)TGTC-3', on the bcrABD promoter that were required for bcrA-lacZ expression. To study BcrR binding to this region, we over-produced BcrR with a C-terminal hexa-histidine tag in Escherichia coli membranes, extracted the protein with n-dodecyl-beta-d-maltoside, and subsequently purified it via Ni(2+)-nitrilotriacetic acid and gel filtration chromatography to apparent homogeneity. Purified BcrR was reconstituted into liposomes, and BcrR binding to bcrABD promoter DNA was analyzed using electrophoretic mobility shift assays. Both inverted repeat sequences were required for BcrR binding, both in the presence and absence of bacitracin. These data demonstrate that membrane-bound BcrR binds specifically to the bcrABD promoter, irrespective of bacitracin concentration. We therefore propose that bacitracin-dependent induction of bcrABD expression by BcrR occurs after DNA binding.

High-resolution structure determination of the CylR2 homodimer using paramagnetic relaxation enhancement and structure-based prediction of molecular alignment

Structure determination of homooligomeric proteins by NMR spectroscopy is difficult due to the lack of chemical shift perturbation data, which is very effective in restricting the binding interface in heterooligomeric systems, and the difficulty of obtaining a sufficient number of intermonomer distance restraints. Here we solved the high-resolution solution structure of the 15.4 kDa homodimer CylR2, the regulator of cytolysin production from Enterococcus faecalis, which deviates by 1.1 angstroms from the previously determined X-ray structure. We studied the influence of different experimental information such as long-range distances derived from paramagnetic relaxation enhancement, residual dipolar couplings, symmetry restraints and intermonomer Nuclear Overhauser Effect restraints on the accuracy of the derived structure. In addition, we show that it is useful to combine experimental information with methods of ab initio docking when the available experimental data are not sufficient to obtain convergence to the correct homodimeric structure. In particular, intermonomer distances may not be required when residual dipolar couplings are compared to values predicted on the basis of the charge distribution and the shape of ab initio docking solutions.

Lantibiotics: peptides of diverse structure and function

The current need for antibiotics with novel target molecules has coincided with advances in technical approaches for the structural and functional analysis of the lantibiotics, which are ribosomally synthesized peptides produced by gram-positive bacteria. These peptides have antibiotic or morphogenetic activity and are structurally defined by the presence of unusual amino acids introduced by posttranslational modification. Lantibiotics are complex polycyclic molecules formed by the dehydration of select Ser and Thr residues and the intramolecular addition of Cys thiols to the resulting unsaturated amino acids to form lanthionine and methyllanthionine bridges, respectively. Importantly, the structural and functional diversity of the lantibiotics is much broader than previously imagined. Here we discuss this growing collection of molecules and introduce some recently discovered peptides, review advances in enzymology and protein engineering, and discuss the regulatory networks that govern the synthesis of the lantibiotics by the producing organisms.

Properties of Enterococcus faecalis plasmid pAD1, a member of a widely disseminated family of pheromone-responding, conjugative, virulence elements encoding cytolysin

The 60-kb pAD1 represents a large and widely disseminated family of conjugative, pheromone-responding, virulence plasmids commonly found in clinical isolates of Enterococcus faecalis. It encodes a hemolysin/bacteriocin (cytolysin) shown to contribute to virulence in animal models, and the related bacteriocin is active against a wide variety of Gram-positive bacteria. This review summarizes what is currently known about the molecular biology of pAD1, including aspects of its cytolytic, UV-resistance, replication, maintenance, and conjugative properties.

The biology of lantibiotics from the lacticin 481 group is coming of age

Lantibiotics are antimicrobial peptides from the bacteriocin family, secreted by Gram-positive bacteria. These peptides differ from other bacteriocins by the presence of (methyl)lanthionine residues, which result from enzymatic modification of precursor peptides encoded by structural genes. Several groups of lantibiotics have been distinguished, the largest of which is the lacticin 481 group. This group consists of at least 16 members, including lacticin 481, streptococcin A-FF22, mutacin II, nukacin ISK-1, and salivaricins. We present the first review devoted to this lantibiotic group, knowledge of which has increased significantly within the last few years. After updating the group composition and defining the common properties of these lantibiotics, we highlight the most recent developments. The latter concern: transcriptional regulation of the lantibiotic genes; understanding the biosynthetic machinery, in particular the ability to perform in vitro prepeptide maturation; characterization of a novel type of immunity protein; and broad application possibilities. This group differs in many aspects from the best known lantibiotic group (nisin group), but shares properties with less-studied groups such as the mersacidin, cytolysin and lactocin S groups.

NMR analysis of protein interactions

Recent technological advances in NMR spectroscopy have alleviated the size limitations for the determination of biomolecular structures in solution. At the same time, novel NMR parameters such as residual dipolar couplings are providing greater accuracy. As this review shows, the structures of protein-protein and protein-nucleic acid complexes up to 50 kDa can now be accurately determined. Although de novo structure determination still requires considerable effort, information on interaction surfaces from chemical shift perturbations is much easier to obtain. Advances in modelling and data-driven docking procedures allow this information to be used for determining approximate structures of biomolecular complexes. As a result, a wealth of information has become available on the way in which proteins interact with other biomolecules. Of particular interest is the fact that these NMR-based methods can be applied to weak and transient protein-protein complexes that are difficult to study by other structural methods.

High-resolution crystal structure of the restriction-modification controller protein C.AhdI from Aeromonas hydrophila

Restriction-modification (R-M) systems serve to protect the host bacterium from invading bacteriophage. The multi-component system includes a methyltransferase, which recognizes and methylates a specific DNA sequence, and an endonuclease which recognises the same sequence and cleaves within or close to this site. The endonuclease will only cleave DNA that is unmethylated at the specific site, thus host DNA is protected while non-host DNA is cleaved. However, following DNA replication, expression of the endonuclease must be delayed until the host DNA is appropriately methylated. In many R-M systems, this regulation is achieved at the transcriptional level via the controller protein, or C-protein. We have solved the first X-ray structure of an R-M controller protein, C.AhdI, to 1.69 A resolution using selenomethionine MAD. C.AhdI is part of a Type IIH R-M system from the pathogen Aeromonas hydrophila. The structure reveals an all-alpha protein that contains a classical helix-turn-helix (HTH) domain and can be assigned to the Xre family of transcriptional regulators. Unlike its monomeric structural homologues, an extended helix generates an interface that results in dimerisation of the free protein. The dimer is electrostatically polarised and a positively charged surface corresponds to the position of the DNA recognition helices of the HTH domain. Comparison with the structure of the lambda cI ternary complex suggests that C.AhdI activates transcription through direct contact with the sigma70 subunit of RNA polymerase.

Data-driven docking for the study of biomolecular complexes

With the amount of genetic information available, a lot of attention has focused on systems biology, in particular biomolecular interactions. Considering the huge number of such interactions, and their often weak and transient nature, conventional experimental methods such as X-ray crystallography and NMR spectroscopy are not sufficient to gain structural insight into these. A wealth of biochemical and/or biophysical data can, however, readily be obtained for biomolecular complexes. Combining these data with docking (the process of modeling the 3D structure of a complex from its known constituents) should provide valuable structural information and complement the classical structural methods. In this review we discuss and illustrate the various sources of data that can be used to map interactions and their combination with docking methods to generate structural models of the complexes. Finally a perspective on the future of this kind of approach is given.