Probing the DNA kink structure induced by the hyperthermophilic chromosomal protein Sac7d

DNA Mimic Foldamer Recognition of a Chromosomal Protein

Helical aromatic oligoamide foldamers bearing anionic side chains that mimic the overall shape and charge surface distribution of DNA were synthesized. Their interactions with chromosomal protein Sac7d, a non-sequence-selective DNA-binder that kinks DNA, were investigated by Surface Plasmon Resonance (SPR), Isothermal Titration Calorimetry (ITC), Circular Dichroism spectroscopy (CD), melting curve analysis, Atomic Force Microscopy (AFM), and Nuclear Magnetic Resonance (NMR), as well as by single crystal X-ray crystallography. The foldamers were shown to bind to Sac7d better than a DNA duplex of comparable length. The interaction is diastereoselective and takes place at the DNA binding site. Crystallography revealed that the DNA mimic foldamers have a binding mode of their own and that they can bind to Sac7d without being kinked.

Enhancing the Features of DNA Mimic Foldamers for Structural Investigations

DNA mimic foldamers based on aromatic oligoamide helices bearing anionic phosphonate side chains have been shown to bind to DNA-binding proteins sometimes orders of magnitude better than DNA itself. Here, we introduce new features in the DNA mimic foldamers to facilitate structural investigations of their interactions with proteins. Thirteen new foldamer sequences have been synthesized and characterized using NMR, circular dichroism, molecular modeling, and X-ray crystallography. The results show that foldamer helix handedness can be quantitatively biased by means of a single stereogenic center, that the foldamer structure can be made C2-symmetrical as in palindromic B-DNA sequences, and that associations between foldamer helices can be promoted utilizing dedicated C-terminal residues that act as sticky ends in B-DNA structures.

Predicting Human Risk with Multidrug Resistant Enterobacter hormaechei MS2 having MCR 9 Gene Isolated from the Feces of Healthy Broiler Through Whole-Genome Sequence-Based Analysis

The zoonotic spread of antimicrobial resistance (AMR) and the associated infections are becoming a major threat to the human population worldwide. Strategies to identify the potential pathogen dissemination by seemingly healthy livestock are at a nascent stage and it is of significant importance to monitor environmental evolution of AMR. In this study, a multidrug resistant strain (MDR) of Enterobacter hormaechei MS2 isolated from the feces of healthy broiler chicken has been characterized by whole-genome sequencing-based method. Here, the isolate was primarily subjected to antimicrobial susceptibility testing followed genome sequencing and analysis. From the antimicrobial susceptibility testing result, the strain was found to be resistant to multiple classes of drugs including the colistin which is an important  last resort drug used to treat infectious diseases. The resistome prediction of genomic data further revealed the presence of 7 perfect and 26 strict hits including those for MCR-9 and FosA2. The pathogenicity prediction has also demonstrated the strain to have the potential to be a human pathogen with 0.72 probability. The phylogenetic analysis has also supported the zoonotic potential of the strain due to its clustering with isolates from both human and livestock-associated host groups. The results of the study suggest the need for a strong surveillance system to identify the opportunistic zoonotic pathogens to prevent a silent AMR menace mediated by them. Carriage of multi-drug resistant strains in the livestock gut microbiome is also a serious concern as it has high AMR transmissibility through contact and supply chain activities.

Sequence-Dependent Kink Formation in Short DNA Loops: Theory and Molecular Dynamics Simulations

Kink formation is essential in highly bent DNA complexed with gene regulatory proteins such as histones to release the bending stress stored within the DNA duplex. Local opening of the double-stranded DNA creates a sharp turn along the specific sequence, which leads to the global bending of the DNA strand. Despite the critical role of kink formation, it is still challenging to predict the position of kink formation for a given DNA sequence. In this study, we propose a theoretical model and perform molecular dynamics simulations to quantify the sequence-dependent kink probability of a strongly bent DNA. By incorporating the elastic bending energy and the sequence-specific thermodynamic parameters, we investigate the importance of the DNA sequence on kink formation. We find that the sequence with TA dinucleotide repeats flanked by GC steps increases the kink propensity by more than an order of magnitude under the same bending stress. The number of base pairs involved in the local opening is found to be coupled with the sequence-specific bubble formation free energy. Our study elucidates the molecular origin of the sequence heterogeneity on kink formation, which is fundamental to understanding protein-DNA recognition.

Mapping of scaffold/matrix attachment regions in human genome: a data mining exercise

Scaffold/matrix attachment regions (S/MARs) are DNA elements that serve to compartmentalize the chromatin into structural and functional domains. These elements are involved in control of gene expression which governs the phenotype and also plays role in disease biology. Therefore, genome-wide understanding of these elements holds great therapeutic promise. Several attempts have been made toward identification of S/MARs in genomes of various organisms including human. However, a comprehensive genome-wide map of human S/MARs is yet not available. Toward this objective, ChIP-Seq data of 14 S/MAR binding proteins were analyzed and the binding site coordinates of these proteins were used to prepare a non-redundant S/MAR dataset of human genome. Along with co-ordinate (location) details of S/MARs, the dataset also revealed details of S/MAR features, namely, length, inter-SMAR length (the chromatin loop size), nucleotide repeats, motif abundance, chromosomal distribution and genomic context. S/MARs identified in present study and their subsequent analysis also suggests that these elements act as hotspots for integration of retroviruses. Therefore, these data will help toward better understanding of genome functioning and designing effective anti-viral therapeutics. In order to facilitate user friendly browsing and retrieval of the data obtained in present study, a web interface, MARome (http://bioinfo.net.in/MARome), has been developed.

New protein-DNA complexes in archaea: a small monomeric protein induces a sharp V-turn DNA structure

MC1, a monomeric nucleoid-associated protein (NAP), is structurally unrelated to other DNA-binding proteins. The protein participates in the genome organization of several Euryarchaea species through an atypical compaction mechanism. It is also involved in DNA transcription and cellular division through unknown mechanisms. We determined the 3D solution structure of a new DNA-protein complex formed by MC1 and a strongly distorted 15 base pairs DNA. While the protein just needs to adapt its conformation slightly, the DNA undergoes a dramatic curvature (the first two bend angles of 55° and 70°, respectively) and an impressive torsional stress (dihedral angle of 106°) due to several kinks upon binding of MC1 to its concave side. Thus, it adopts a V-turn structure. For longer DNAs, MC1 stabilizes multiple V-turn conformations in a flexible and dynamic manner. The existence of such V-turn conformations of the MC1-DNA complexes leads us to propose two binding modes of the protein, as a bender (primary binding mode) and as a wrapper (secondary binding mode). Moreover, it opens up new opportunities for studying and understanding the repair, replication and transcription molecular machineries of Archaea.

Extensive drug resistant Salmonella enterica serovar Senftenberg carrying blaNDM encoding plasmid p5558 (IncA/C) from India

Non-typhoidal Salmonella (NTS) are foodborne pathogens that are responsible for self-limiting gastroenteritis in humans. The present study aims at the molecular characterisation and comparative genomics of Salmonella enterica serovar Senftenberg strain P5558 isolated from the pus samples of a patient suffering from stump infection. The isolate was subjected to serotyping and antimicrobial susceptibility test to understand the phenotypical characteristics. Whole genome sequencing (WGS) was carried out and comparative genomics using computational tools showed the antimicrobial resistance and virulence gene profile of the isolates from the genome sequence data. Typing experiments confirmed that the isolate belong to S. Senftenberg with sequence type ST14. Resistance against β-lactams is associated with the presence of bla_TEM-1, bla_OXA-9, bla_CMY-2 and bla_NDM-1 genes. Similarly resistance to aminoglycoside was associated with five aminoglycoside modifying enzymes aac(6')-Ia, aac(6')-Ib, aph(3')-Ib, aph(6')-Ib and ant(3'')-Ia, sulfonamide with sul-1 and sul-2 and chloramphenicol with florR gene. Substitutions in gyrA (S83Y, D87G) and parC (S80I) genes found to be the reason for fluoroquinolone resistance. The plasmid profiling showed the isolate has four resistance plasmids in which plasmid p5558-NDM (IncA/C) harbours major resistance genes including bla_NDM-1 and bla_CMY-2. Determination of virulence gene profile revealed that the genome carries all major Salmonella pathogenicity islands and virulence factors. From our findings it is clear that the isolate possess characteristic pathogenicity islands (SPI 1-6, 13, 14), major virulence factors and acquired resistance genes. Comparative analysis suggests the evolution and distribution of the MDR gene encoding plasmids in NTS.

Strong deformations of DNA: Effect on the persistence length

Extreme deformations of the DNA double helix attracted a lot of attention during the past decades. Particularly, the determination of the persistence length of DNA with extreme local disruptions, or kinks, has become a crucial problem in the studies of many important biological processes. In this paper we review an approach to calculate the persistence length of the double helix by taking into account the formation of kinks of arbitrary configuration. The reviewed approach improves the Kratky-Porod model to determine the type and nature of kinks that occur in the double helix, by measuring a reduction of the persistence length of the kinkable DNA.

Twist and turn: a revised structural view on the unpaired bubble of class II CPD photolyase in complex with damaged DNA

Cyclobutane pyrimidine dimer (CPD) photolyases harness the energy of blue light to repair UV-induced DNA CPDs. Upon binding, CPD photolyases cause the photodamage to flip out of the duplex DNA and into the catalytic site of the enzyme. This process, called base-flipping, induces a kink in the DNA, as well as an unpaired bubble, which are stabilized by a network of protein-nucleic acid interactions. Previously, several co-crystal structures have been reported in which the binding mode of CPD photolyases has been studied in detail. However, in all cases the internucleoside linkage of the photodamage site was a chemically synthesized formacetal analogue and not the natural phosphodiester. Here, the first crystal structure and conformational analysis via molecular-dynamics simulations of a class II CPD photolyase in complex with photodamaged DNA that contains a natural cyclobutane pyrimidine dimer with an intra-lesion phosphodiester linkage are presented. It is concluded that a highly conserved bubble-intruding region (BIR) mediates stabilization of the open form of CPD DNA when complexed with class II CPD photolyases.

Probing the role of intercalating protein sidechains for kink formation in DNA

Protein binding can induce DNA kinks, which are for example important to enhance the specificity of the interaction and to facilitate the assembly of multi protein complexes. The respective proteins frequently exhibit amino acid sidechains that intercalate between the DNA base steps at the site of the kink. However, on a molecular level there is only little information available about the role of individual sidechains for kink formation. To unravel structural principles of protein-induced DNA kinking we have performed molecular dynamics (MD) simulations of five complexes that varied in their architecture, function, and identity of intercalated residues. Simulations were performed for the DNA complexes of wildtype proteins (Sac7d, Sox-4, CcpA, TFAM, TBP) and for mutants, in which the intercalating residues were individually or combined replaced by alanine. The work revealed that for systems with multiple intercalated residues, not all of them are necessarily required for kink formation. In some complexes (Sox-4, TBP), one of the residues proved to be essential for kink formation, whereas the second residue has only a very small effect on the magnitude of the kink. In other systems (e.g. Sac7d) each of the intercalated residues proved to be individually capable of conferring a strong kink suggesting a partially redundant role of the intercalating residues. Mutation of the key residues responsible for kinking either resulted in stable complexes with reduced kink angles or caused conformational instability as evidenced by a shift of the kink to an adjacent base step. Thus, MD simulations can help to identify the role of individual inserted residues for kinking, which is not readily apparent from an inspection of the static structures. This information might be helpful for understanding protein-DNA interactions in more detail and for designing proteins with altered DNA binding properties in the future.

Roles of Leu28 side chain intercalation in the interaction between Cren7 and DNA

Crenarchaeal chromatin protein Cren7 binds double-stranded DNA in the minor groove, introducing a sharp single-step DNA kink. The side chain of Leu28, a residue conserved among all Cren7 homologs, intercalates into the kinked DNA step. In the present study, we replaced Leu28 with a residue containing a hydrophobic side chain of different sizes (i.e. L28A, L28V, L28I, L28M and L28F). Both the stability of the Cren7-DNA complex and the ability of Cren7 to constrain DNA supercoils correlated well with the size of the intercalated side chain. Structural analysis shows that L28A induces a kink (∼43°), nearly as sharp as that produced by wild-type Cren7 (∼48°), in the bound DNA fragment despite the lack of side chain intercalation. In another duplex DNA fragment, L28F inserts a large hydrophobic side chain deep into the DNA step, but introduces a smaller kink (∼39°) than that formed by the wild-type protein (∼50°). Mutation of Leu28 into methionine yields two protein conformers differing in loop β3-β4 orientation, DNA-binding surface and DNA geometry in the protein-DNA structure. Our results indicate that side chain intercalation is not directly responsible for DNA kinking or bending by Cren7, but plays a critical role in the stabilization of the Cren7-DNA complex. In addition, the flexibility of loop β3-β4 in Cren7, as revealed in the crystal structure of L28M-DNA, may serve a role in the modulation of chromosomal organization and function in the cell.

The Arginine Pairs and C-Termini of the Sso7c4 from Sulfolobus solfataricus Participate in Binding and Bending DNA

The Sso7c4 from Sulfolobus solfataricus forms a dimer, which is believed to function as a chromosomal protein involved in genomic DNA compaction and gene regulation. Here, we present the crystal structure of wild-type Sso7c4 at a high resolution of 1.63 Å, showing that the two basic C-termini are disordered. Based on the fluorescence polarization (FP) binding assay, two arginine pairs, R11/R22' and R11'/R22, on the top surface participate in binding DNA. As shown in electron microscopy (EM) images, wild-type Sso7c4 compacts DNA through bridging and bending interactions, whereas the binding of C-terminally truncated proteins rigidifies and opens DNA molecules, and no compaction of the DNA occurs. Moreover, the FP, EM and fluorescence resonance energy transfer (FRET) data indicated that the two basic and flexible C-terminal arms of the Sso7c4 dimer play a crucial role in binding and bending DNA. Sso7c4 has been classified as a repressor-like protein because of its similarity to Escherichia coli Ecrep 6.8 and Ecrep 7.3 as well as Agrobacterium tumefaciens ACCR in amino acid sequence. Based on these data, we proposed a model of the Sso7c4-DNA complex using a curved DNA molecule in the catabolite activator protein-DNA complex. The DNA end-to-end distance measured with FRET upon wild-type Sso7c4 binding is almost equal to the distance measured in the model, which supports the fidelity of the proposed model. The FRET data also confirm the EM observation showing that the binding of wild-type Sso7c4 reduces the DNA length while the C-terminal truncation does not. A functional role for Sso7c4 in the organization of chromosomal DNA and/or the regulation of gene expression through bridging and bending interactions is suggested.

Whole-genome sequencing and comparative genomic analysis of Escherichia coli O91 strains isolated from symptomatic and asymptomatic human carriers

Background

The Shiga toxin–producing Escherichia coli (STEC) O91:H21 strains NCCP15736 and NCCP15737 were isolated during a single outbreak in Korea, NCCP15736 from a symptomatic carrier and NCCP15737 from an asymptomatic carrier. To investigate genomic differences between the two strains, we performed whole-genome sequencing of both strains and conducted a comparative genomic analysis.

Results

Using the Illumina HiSeq 2000 platform and Rapid Annotation using the Subsystem Technology (RAST) server, whole-genome sequences of NCCP15736 and NCCP15737 were obtained and annotated. Phylogenetic analysis of ten E. coli strains showed that NCCP15736 and NCCP15737 are evolutionarily close. The two strains were found to be most close to E. coli O91:NM str. 2009C-3745. The genomic comparison showed that the fimD gene of NCCP15737 is truncated and that the truncation could underlie the defects in infection and pathogenicity of NCCP15737. The two strains showed the same virulence factor profiles, and we identified 25 virulence factors from NCCP15736 and NCCP15737, respectively. We identified ten and nine phage-associated regions in the NCCP15736 and NCCP15737 genomes, respectively; the two strains share five of these.

Conclusions

NCCP15736 and NCCP15737 differ at the genomic level, even though they share features such as virulence-related genes. NCCP15737 has a deletion in fimD, which may underlie its asymptomatic character. We conclude that complete genome sequencing and integration of other types of omics data are needed to fully reveal the mechanism underlying the asymptomatic character of NCCP15737.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-016-0138-9) contains supplementary material, which is available to authorized users.

Molecular analysis for screening human bacterial pathogens in municipal wastewater treatment and reuse

Effective and sensitive monitoring of human pathogenic bacteria in municipal wastewater treatment is important not only for managing public health risk related to treated wastewater reuse, but also for ensuring proper functioning of the treatment plant. In this study, three different 16S rRNA gene molecular analysis methodologies were employed to screen bacterial pathogens in samples collected at three different stages of an activated sludge plant. Overall bacterial diversity was analyzed using next generation sequencing (NGS) on the Illumina MiSeq platform, as well as PCR-DGGE followed by band sequencing. In addition, a microdiversity analysis was conducted using PCR-DGGE, targeting Escherichia coli. Bioinformatics analysis was performed using QIIME protocol by clustering sequences against the Human Pathogenic Bacteria Database. NGS data were also clustered against the Greengenes database for a genera-level diversity analysis. NGS proved to be the most effective approach screening the sequences of 21 potential human bacterial pathogens, while the E. coli microdiversity analysis yielded one (O157:H7 str. EDL933) out of the two E. coli strains picked up by NGS. Overall diversity using PCR-DGGE did not yield any pathogenic sequence matches even though a number of sequences matched the NGS results. Overall, sequences of Gram-negative pathogens decreased in relative abundance along the treatment train while those of Gram-positive pathogens increased.

FixingTIM: interactive exploration of sequence and structural data to identify functional mutations in protein families

BACKGROUND

Knowledge of the 3D structure and functionality of proteins can lead to insight into the associated cellular processes, speed up the creation of pharmaceutical products, and develop drugs that are more effective in combating disease.

METHODS

We present the design and implementation of a visual mining and analysis tool to help identify protein mutations across a family of structural models and to help discover the effect of these mutations on protein function. We integrate 3D structure and sequence information in a common visual interface; multiple linked views and a computational backbone allow comparison at the molecular and atomic levels, while a novel trend-image visual abstraction allows for the sorting and mining of large collections of sequences and of their residues.

RESULTS

We evaluate our approach on the triosephosphate isomerase (TIM) family structural models and sequence data and show that our tool provides an effective, scalable way to navigate a family of proteins, as well as a means to inspect the structure and sequence of individual proteins.

CONCLUSIONS

The TIM application shows that our tool can assist in the navigation of families of proteins, as well as in the exploration of individual protein structures. In conjunction with domain expert knowledge, this interactive tool can help provide biophysical insight into why specific mutations affect function and potentially suggest additional modifications to the protein that could be used to rescue functionality.

Model of a DNA-protein complex of the architectural monomeric protein MC1 from Euryarchaea

In Archaea the two major modes of DNA packaging are wrapping by histone proteins or bending by architectural non-histone proteins. To supplement our knowledge about the binding mode of the different DNA-bending proteins observed across the three domains of life, we present here the first model of a complex in which the monomeric Methanogen Chromosomal protein 1 (MC1) from Euryarchaea binds to the concave side of a strongly bent DNA. In laboratory growth conditions MC1 is the most abundant architectural protein present in Methanosarcina thermophila CHTI55. Like most proteins that strongly bend DNA, MC1 is known to bind in the minor groove. Interaction areas for MC1 and DNA were mapped by Nuclear Magnetic Resonance (NMR) data. The polarity of protein binding was determined using paramagnetic probes attached to the DNA. The first structural model of the DNA-MC1 complex we propose here was obtained by two complementary docking approaches and is in good agreement with the experimental data previously provided by electron microscopy and biochemistry. Residues essential to DNA-binding and -bending were highlighted and confirmed by site-directed mutagenesis. It was found that the Arg25 side-chain was essential to neutralize the negative charge of two phosphates that come very close in response to a dramatic curvature of the DNA.

DNA–RNA hybrid duplexes with decreasing pyrimidine content in the DNA strand provide structural snapshots for the A- to B-form conformational transition of nucleic acids

A gradual increase in the deoxypyrimidine content in DNA–RNA hybrids leads to B- to A-form nucleic acid transition. Possible factors that govern nuclease activity on hybrid duplexes are presented.

Tolerance of the archaeal Sac7d scaffold protein to alternative library designs: characterization of anti-immunoglobulin G Affitins

Engineered protein scaffolds have received considerable attention as alternatives to antibodies in both basic and applied research, as they can offer superior biophysical properties often associated with a simpler molecular organization. Sac7d has been demonstrated as an effective scaffold for molecular recognition. Here, we used the initial L1 'flat surface' library constructed by randomization of 14 residues, to identify ligands specific for human immunoglobulin G. To challenge the plasticity of the Sac7d protein scaffold, we designed the alternative L2 'flat surface & loops' library whereof only 10 residues are randomized. Representative binders (Affitins) of the two libraries exhibited affinities in the low nanomolar range and were able to recognize different epitopes within human immunoglobulin G. These Affitins were stable up to pH 12 while largely conserving other favorable properties of Sac7d protein, such as high expression yields in Escherichia coli, solubility, thermal stability up to 80.7°C, and acidic stability (pH 0). In agreement with our library designs, mutagenesis study revealed two distinct binding areas, one including loops. Together, our results indicate that the Sac7d scaffold tolerates alternative library designs, which further expands the diversity of Affitins and may provide a general way to create tailored affinity tools for demanding applications.

An ensemble of B-DNA dinucleotide geometries lead to characteristic nucleosomal DNA structure and provide plasticity required for gene expression

BACKGROUND

A nucleosome is the fundamental repeating unit of the eukaryotic chromosome. It has been shown that the positioning of a majority of nucleosomes is primarily controlled by factors other than the intrinsic preference of the DNA sequence. One of the key questions in this context is the role, if any, that can be played by the variability of nucleosomal DNA structure.

RESULTS

In this study, we have addressed this question by analysing the variability at the dinucleotide and trinucleotide as well as longer length scales in a dataset of nucleosome X-ray crystal structures. We observe that the nucleosome structure displays remarkable local level structural versatility within the B-DNA family. The nucleosomal DNA also incorporates a large number of kinks.

CONCLUSIONS

Based on our results, we propose that the local and global level versatility of B-DNA structure may be a significant factor modulating the formation of nucleosomes in the vicinity of high-plasticity genes, and in varying the probability of binding by regulatory proteins. Hence, these factors should be incorporated in the prediction algorithms and there may not be a unique 'template' for predicting putative nucleosome sequences. In addition, the multimodal distribution of dinucleotide parameters for some steps and the presence of a large number of kinks in the nucleosomal DNA structure indicate that the linear elastic model, used by several algorithms to predict the energetic cost of nucleosome formation, may lead to incorrect results.

Molecular simulations on the thermal stabilization of DNA by hyperthermophilic chromatin protein Sac7d, and associated conformational transitions

Sac7d belongs to a family of chromosomal proteins, which are crucial for thermal stabilization of DNA at higher growth temperatures. It is capable of binding DNA nonspecifically, and is responsible for the increase in the melting temperature of DNA in the bound form up to 85 °C. Molecular dynamics (MD) simulations were performed at different temperatures on two protein-DNA complexes of Sac7d. Various structural and energetic parameters were calculated to examine the DNA stability and to investigate the conformational changes in DNA and the protein-DNA interactions. Room temperature simulations indicated very good agreement with the experimental structures. The protein structure is nearly unchanged at both 300 and 360 K, and only up to five base pairs of the DNA are stabilized by Sac7d at 360 K. However, the MD simulations on DNA alone systems show that they lose their helical structures at 360 K further supporting the role of Sac7d in stabilizing the oligomers. At higher temperatures (420 and 480 K), DNA undergoes denaturation in the presence and the absence of the protein. The DNA molecules were found to undergo B- to A-form transitions consistent with experimental studies, and the extent of these transitions are examined in detail. The extent of sampling B- and A-form regions was found to show temperature and sequence dependence. Multiple MD simulations yielded similar results validating the proposed model. Interaction energy calculations corresponding to protein-DNA binding indicates major contribution due to DNA backbone, explaining the nonspecific interactions of Sac7d.

Crystal structure of the crenarchaeal conserved chromatin protein Cren7 and double-stranded DNA complex

Cren7 is a crenarchaeal conserved chromatin protein discovered recently. To explore the mechanism of the DNA packaging in Crenarchaeota, the crystal structure of Cren7-GCGATCGC complex has been determined and refined at 1.6 A resolution. Cren7 kinks the dsDNA sharply similar to Sul7d, another chromatin protein existing only in Sulfolobales, which reveals that the "bending and unwinding" compacting mechanism is conserved in Crenarchaeota. Significant structural differences are revealed by comparing both protein-dsDNA complexes. The kinked sites on the same dsDNA in the complexes with Sul7d and Cren7 show one base pair shift. For Cren7, fewer charged residues in the beta-barrel structural region bind to DNA, and additionally, the flexible loop L(beta3beta4) is also involved in the binding. Electrophoretic mobility shift assays indicate that loop L(beta3beta4) is essential for DNA-binding of Cren7. These differences provide insight into the functional difference of both chromatin proteins, suggesting that Cren7 may be more regulative than Sul7d in the DNA-binding affinity by the methylation in the flexible loop L(beta3beta4) in vivo.

Structural insights into the interaction of the crenarchaeal chromatin protein Cren7 with DNA

Cren7, a newly found chromatin protein, is highly conserved in the Crenarchaeota. The protein shows higher affinity for double-stranded DNA than for single-stranded DNA, constrains negative DNA supercoils in vitro and is associated with genomic DNA in vivo. Here we report the crystal structures of the Cren7 protein from Sulfolobus solfataricus in complex with two DNA sequences. Cren7 binds in the minor groove of DNA and causes a single-step sharp kink in DNA (approximately 53 degrees) through the intercalation of the hydrophobic side chain of Leu28. Loop beta 3-beta 4 of Cren7 undergoes a significant conformational change upon binding of the protein to DNA, suggesting its critical role in the stabilization of the protein-DNA complex. The roles of DNA-contacting amino acid residues in stabilizing the Cren7-DNA interaction were examined by mutational analysis. Structural comparison of Cren7-DNA complexes with Sac7d-DNA complexes reveals significant differences between the two proteins in DNA binding surface, suggesting that Cren7 and Sul7d serve distinct functions in chromosomal organization.

Structural analysis of Rtt106p reveals a DNA binding role required for heterochromatin silencing

Rtt106p is a Saccharomyces cerevisiae histone chaperone with roles in heterochromatin silencing and nucleosome assembly. The molecular mechanism by which Rtt106p engages in chromatin dynamics remains unclear. Here, we report the 2.5 A crystal structure of the core domain of Rtt106p, which adopts an unusual "double pleckstrin homology" domain architecture that represents a novel structural mode for histone chaperones. A histone H3-H4-binding region and a novel double-stranded DNA-binding region have been identified. Mutagenesis studies reveal that the histone and DNA binding activities of Rtt106p are involved in Sir protein-mediated heterochromatin formation. Our results uncover the structural basis of the diverse functions of Rtt106p and provide new insights into its cellular roles.

Regional mutagenesis of the gene encoding the phage Mu late gene activator C identifies two separate regions important for DNA binding

Lytic development of bacteriophage Mu is controlled by a regulatory cascade and involves three phases of transcription: early, middle and late. Late transcription requires the host RNA polymerase holoenzyme and a 16.5-kDa Mu-encoded activator protein C. Consistent with these requirements, the four late promoters P_lys, P_I, P_P and P_mom have recognizable −10 hexamers but lack typical −35 hexamers. The C protein binds to a 16-bp imperfect dyad-symmetrical sequence element centered at −43.5 and overlapping the −35 region. Based on the crystal structure of the closely related Mor protein, the activator of Mu middle transcription, we predict that two regions of C are involved in DNA binding: a helix-turn-helix region and a β-strand region linking the dimerization and helix-turn-helix domains. To test this hypothesis, we carried out mutagenesis of the corresponding regions of the C gene by degenerate oligonucleotide-directed PCR and screened the resulting mutants for their ability to activate a P_lys-galK fusion. Analysis of the mutant proteins by gel mobility shift, β-galactosidase and polyacrylamide gel electrophoresis assays identified a number of amino acid residues important for C DNA binding in both regions.

Biochemical and structural characterization of Cren7, a novel chromatin protein conserved among Crenarchaea

Archaea contain a variety of chromatin proteins consistent with the evolution of different genome packaging mechanisms. Among the two main kingdoms in the Archaea, Euryarchaeota synthesize histone homologs, whereas Crenarchaeota have not been shown to possess a chromatin protein conserved at the kingdom level. We report the identification of Cren7, a novel family of chromatin proteins highly conserved in the Crenarchaeota. A small, basic, methylated and abundant protein, Cren7 displays a higher affinity for double-stranded DNA than for single-stranded DNA, constrains negative DNA supercoils and is associated with genomic DNA in vivo. The solution structure and DNA-binding surface of Cren7 from the hyperthermophilic crenarchaeon Sulfolobus solfataricus were determined by NMR. The protein adopts an SH3-like fold. It interacts with duplex DNA through a beta-sheet and a long flexible loop, presumably resulting in DNA distortions through intercalation of conserved hydrophobic residues into the DNA structure. These data suggest that the crenarchaeal kingdom in the Archaea shares a common strategy in chromatin organization.

DISPLAR: an accurate method for predicting DNA-binding sites on protein surfaces

Structural and physical properties of DNA provide important constraints on the binding sites formed on surfaces of DNA-targeting proteins. Characteristics of such binding sites may form the basis for predicting DNA-binding sites from the structures of proteins alone. Such an approach has been successfully developed for predicting protein-protein interface. Here this approach is adapted for predicting DNA-binding sites. We used a representative set of 264 protein-DNA complexes from the Protein Data Bank to analyze characteristics and to train and test a neural network predictor of DNA-binding sites. The input to the predictor consisted of PSI-blast sequence profiles and solvent accessibilities of each surface residue and 14 of its closest neighboring residues. Predicted DNA-contacting residues cover 60% of actual DNA-contacting residues and have an accuracy of 76%. This method significantly outperforms previous attempts of DNA-binding site predictions. Its application to the prion protein yielded a DNA-binding site that is consistent with recent NMR chemical shift perturbation data, suggesting that it can complement experimental techniques in characterizing protein-DNA interfaces.

What drives proteins into the major or minor grooves of DNA?

The energetic profiles of a significant number of protein-DNA systems at 20 degrees C reveal that, despite comparable Gibbs free energies, association with the major groove is primarily an enthalpy-driven process, whereas binding to the minor groove is characterized by an unfavorable enthalpy that is compensated by favorable entropic contributions. These distinct energetic signatures for major versus minor groove binding are irrespective of the magnitude of DNA bending and/or the extent of binding-induced protein refolding. The primary determinants of their different energetic profiles appear to be the distinct hydration properties of the major and minor grooves; namely, that the water in the A+T-rich minor groove is in a highly ordered state and its removal results in a substantial positive contribution to the binding entropy. Since the entropic forces driving protein binding into the minor groove are a consequence of displacing water ordered by the regular arrangement of polar contacts, they cannot be regarded as hydrophobic.