DNA-induced conformational changes in bacteriophage 434 repressor

A quantitative characterization of interaction between prion protein with nucleic acids

Binding of recombinant prion protein with small highly structured RNAs, prokaryotic and eukaryotic prion protein mRNA pseudoknots, tRNA and polyA has been studied by the change in fluorescence anisotropy of the intrinsic tryptophan groups of the protein. The affinities of these RNAs to the prion protein and the number of sites where the protein binds to the nucleic acids do not vary appreciably although the RNAs have very different compositions and structures. The binding parameters do not depend upon pH of the solution and show a poor co-operativity. The reactants form larger nucleoprotein complexes at pH 5 compared to that at neutral pH. The electrostatic force between the protein and nucleic acids dominates the binding interaction at neutral pH. In contrast, nucleic acid interaction with the incipient nonpolar groups exposed from the structured region of the prion protein dominates the reaction at pH 5. Prion protein of a particular species forms larger complexes with prion protein mRNA pseudoknots of the same species. The structure of the pseudoknots and not their base sequences probably dominates their interaction with prion protein. Possibilities of the conversion of the prion protein to its infectious form in the cytoplasm by nucleic acids have been discussed.

Determinants of bacteriophage 933W repressor DNA binding specificity

We reported previously that 933W repressor apparently does not cooperatively bind to adjacent sites on DNA and that the relative affinities of 933W repressor for its operators differ significantly from that of any other lambdoid bacteriophage. These findings indicate that the operational details of the lysis-lysogeny switch of bacteriophage 933W are unique among lambdoid bacteriophages. Since the functioning of the lysis-lysogeny switch in 933W bacteriophage uniquely and solely depends on the order of preference of 933W repressor for its operators, we examined the details of how 933W repressor recognizes its DNA sites. To identify the specificity determinants, we first created a molecular model of the 933W repressor-DNA complex and tested the predicted protein-DNA interactions. These results of these studies provide a picture of how 933W repressor recognizes its DNA sites. We also show that, opposite of what is normally observed for lambdoid phages, 933W operator sequences have evolved in such a way that the presence of the most commonly found base sequences at particular operator positions serves to decrease, rather than increase, the affinity of the protein for the site. This finding cautions against assuming that a consensus sequence derived from sequence analysis defines the optimal, highest affinity DNA binding site for a protein.

Modeling of the genetic switch of bacteriophage TP901-1: A heteromer of CI and MOR ensures robust bistability

The lytic-lysogenic switch of the temperate lactococcal phage TP901-1 is fundamentally different from that of phage lambda. In phage TP901-1, the lytic promoter P(L) is repressed by CI, whereas repression of the lysogenic promoter P(R) requires the presence of both of the antagonistic regulator proteins, MOR and CI. We model the central part of the switch and compare the two cases for P(R) repression: the one where the two regulators interact only on the DNA and the other where the two regulators form a heteromer complex in the cytoplasm prior to DNA binding. The models are analyzed for bistability, and the predicted promoter repression folds are compared to experimental data. We conclude that the experimental data are best reproduced the latter case, where a heteromer complex forms in solution. We further find that CI sequestration by the formation of MOR:CI complexes in cytoplasm makes the genetic switch robust.

The bacteriophage 434 repressor dimer preferentially undergoes autoproteolysis by an intramolecular mechanism

Inactivation of the lambdoid phage repressor protein is necessary to induce lytic growth of a lambdoid prophage. Activated RecA, the mediator of the host SOS response to DNA damage, causes inactivation of the repressor by stimulating the repressor's nascent autocleavage activity. The repressor of bacteriophage lambda and its homolog, LexA, preferentially undergo RecA-stimulated autocleavage as free monomers, which requires that each monomer mediates its own (intramolecular) cleavage. The cI repressor of bacteriophage 434 preferentially undergoes autocleavage as a dimer specifically bound to DNA, opening the possibility that one 434 repressor subunit may catalyze proteolysis of its partner subunit (intermolecular cleavage) in the DNA-bound dimer. Here, we first identified and mutagenized the residues at the cleavage and active sites of 434 repressor. We utilized the mutant repressors to show that the DNA-bound 434 repressor dimer overwhelmingly prefers to use an intramolecular mechanism of autocleavage. Our data suggest that the 434 repressor cannot be forced to use an intermolecular cleavage mechanism. Based on these data, we propose a model in which the cleavage-competent conformation of the repressor is stabilized by operator binding.

Purification and characterization of the repressor of the shiga toxin-encoding bacteriophage 933W: DNA binding, gene regulation, and autocleavage

The genes encoding Shiga toxin (stx), the major virulence factor of Shiga toxin-encoding Escherichia coli (STEC) strains, are carried on lambdoid prophages resident in all known STEC strains. The stx genes are expressed only during lytic growth of these temperate bacteriophages. We cloned the gene encoding the repressor of the Shiga toxin-encoding bacteriophage 933W and examined the DNA binding and transcriptional regulatory activities of the overexpressed, purified protein. Typical of nearly all lambdoid phage repressors, 933W repressor binds to three sites in 933W right operator (OR). Also typical, when bound at OR, 933W repressor functions as an activator at the PRM promoter and a repressor at the PR promoter. In contrast to other lambdoid bacteriophages, 933W left operator (OL) contains only two repressor binding sites, but the OL-bound repressor still efficiently represses PL transcription. Lambdoid prophage induction requires inactivation of the repressor's DNA binding activity. In all phages examined thus far, this inactivation requires a RecA-stimulated repressor autoproteolysis event, with cleavage occurring precisely in an Ala-Gly dipeptide sequence that is found within a "linker " region that joins the two domains of these proteins. However, 933W repressor protein contains neither an Ala-Gly nor an alternative Cys-Gly dipeptide cleavage site anywhere in its linker sequence. We show here that the autocleavage occurs at a Leu-Gly dipeptide. Thus, the specificity of the repressor autocleavage site is more variable than thought previously.

Nucleic Acid and Prion Protein Interaction Produces Spherical Amyloids which can Function in vivo as Coats of Spongiform Encephalopathy Agent

The infectious agent of transmissible spongiform encephalopathies (TSE) has been considered to be PrP(SC), a structural isoform of cellular prion protein PrP(C). PrP(SC) can exist as oligomers and/or as amyloid polymers. Nucleic acids induce structural conversion of recombinant prion protein PrP and PrP(C) to PrP(SC) form in solution and in vitro. Here, we report that nucleic acids, by interacting with PrP in solution, produce amyloid fibril and fibres of different morphologies, similar to those identified in the diseased brains. In addition, the same interaction produces polymer lattices and spherical amyloids of different dimensions (15-150 nm in diameters). The polymer lattices show apparent morphological similarity to the two-dimensional amyloid crystals obtained from linear amyloids isolated in vivo. The spherical amyloids structurally resemble "spherical particles" observed in natural spongiform encephalopathy (SE) and in scrapie-infected brains (TSE). We suggest that spherical amyloids, PrP(SC)-amylospheroids, are probable constituents of the coat of the spherical particles found in vivo and the latter can act as protective coats of the SE and TSE agents in vivo.

DNA-mediated assembly of weakly interacting DNA-binding protein subunits: in vitro recruitment of phage 434 repressor and yeast GCN4 DNA-binding domains

The specificity of DNA-mediated protein assembly was studied in two in vitro systems, based on (i) the DNA-binding domain of bacteriophage 434 repressor cI (amino acid residues 1-69), or (ii) the DNA-binding domain of the yeast transcription factor GCN4, (amino acids 1-34) and their respective oligonucleotide cognates. In vivo, both of these peptides are part of larger protein molecules that also contain dimerization domains, and the resulting dimers recognize cognate palindromic DNA sequences that contain two half-sites of 4 bp each. The dimerization domains were not included in the peptides tested, so in solution-in the presence or absence of non-cognate DNA oligonucleotides-these molecules did not show appreciable dimerization, as determined by pyrene excimer fluorescence spectroscopy and oxidative cross-linking monitored by mass spectrometry. Oligonucleotides with only one 4 bp cognate half-site were able to initiate measurable dimerization, and two half-sites were able to select specific dimers even from a heterogeneous pool of molecules of closely related specificity (such as DNA-binding domains of the 434 repressor and their engineered mutants that mimic the binding helix of the related P22 phage repressor). The fluorescent technique allowed us to separately monitor the unspecific, ionic interaction of the peptides with DNA which produced a roughly similar signal in the case of both cognate and non-cognate oligonucleotides. But in the former case, a concomitant excimer fluorescence signal showed the formation of correctly positioned dimers. The results suggest that DNA acts as a highly specific template for the recruitment of weakly interacting protein molecules that can thus build up highly specific complexes.

Double-stranded DNA-induced localized unfolding of HCV NS3 helicase subdomain 2

The NS3 helicase of the hepatitis C virus (HCV) unwinds double-stranded (ds) nucleic acid (NA) in an NTP-dependent fashion. Mechanistic details of this process are, however, largely unknown for the HCV helicase. We have studied the binding of dsDNA to an engineered version of subdomain 2 of the HCV helicase (d(2Delta)NS3h) by NMR and circular dichroism. Binding of dsDNA to d(2Delta)NS3h induces a local unfolding of helix (alpha(3)), which includes residues of conserved helicase motif VI (Q(460)RxxRxxR(467)), and strands (beta(1) and beta(8)) from the central beta-sheet. This also occurs upon lowering the pH (4.4) and introducing an R461A point mutation, which disrupt salt bridges with Asp 412 and Asp 427 in the protein structure. NMR studies on d(2Delta)NS3h in the partially unfolded state at low pH map the dsDNA binding site to residues previously shown to be involved in single-stranded DNA binding. Sequence alignment and structural comparison suggest that these Arg-Asp interactions are highly conserved in SF2 DEx(D/H) proteins. Thus, modulation of these interactions by dsNA may allow SF2 helicases to switch between conformations required for helicase function.

Monovalent Cations Regulate DNA Sequence Recognition by 434 Repressor

The bacteriophage 434 repressor distinguishes between its six naturally occurring binding sites using indirect readout. In indirect readout, sequence-dependent differences in the structure and flexibility of non-contacted bases in a protein's DNA-binding site modulate the affinity of DNA for protein. The conformation and flexibility of a DNA sequence can be influenced by the interaction of the DNA bases or backbone with solution components. We examined the effect of changing the cation-type present in solution on the stability and structure of 434 repressor complexes with wild-type and mutant OR1 and OR3, binding sites that differ in their contacted and non-contacted base sequences. We find that the affinity of repressor for OR1, but not for OR3, depends remarkably on the type and concentration of monovalent cation. Moreover, the formation of a stable, specific repressor-OR1 complex requires the presence of monovalent cations; however, repressor-OR3 complex formation has no such requirement. Changing monovalent cation type alters the ability of repressor to protect OR1, but not OR3, from *OH radical cleavage. Altering the relative length of the poly(dA) x poly(dT) tract in the non-contacted regions of the OR1 and OR3 can reverse the cation sensitivity of repressor's affinities for these two sites. Taken together these findings show that cation-dependent alterations in DNA structure underlies indirect readout of DNA sequence by 434 repressor and perhaps other proteins.

The preferred substrate for RecA-mediated cleavage of bacteriophage 434 repressor is the DNA-bound dimer

Induction of a lysogen of a lambdoid bacteriophage usually involves RecA-stimulated autoproteolysis of the bacteriophage repressor protein. Previous work on the phage repressors showed that the monomeric form of the protein is the target of RecA. Our previous work indicated that in the case of bacteriophage 434, virtually none of the repressor is present as a monomer in vivo. Hence, if the repressor in a lysogen is present as a dimer, how can RecA-stimulated autoproteolysis play a role in bacteriophage induction? We examined this question by determining the rate of RecA-stimulated 434 repressor cleavage as a function of repressor concentration and added DNA. Our results show that binding of 434 repressor to a specific DNA binding site dramatically increases the velocity of repressor autocleavage compared to the velocity of cleavage of the monomer and concentration-induced dimer. DNA binding-deficient hemidimers formed between the intact repressor and its C-terminal domain fragment have a lower rate of cleavage than DNA-bound dimers. These results show that the DNA-bound 434 repressor dimer, which is the form of the repressor that is required for its transcriptional regulatory functions, is the preferred form for RecA-stimulated autocleavage. We also show that the rate of repressor autocleavage is influenced by the sequence of the bound DNA. Kinetic analysis of the autocleavage reaction indicated that the DNA sequence influences the velocity of 434 repressor autocleavage by affecting the affinity of the repressor-DNA complex for RecA, not the chemical cleavage step. Regardless of the mechanism, the finding that the presence and precise sequence of DNA modulate the autocleavage reaction shows that DNA allosterically affects the function of 434 repressor.

Prions at the crossroads: the need to identify the active TSE agent

Structural change in the cellular prion protein, PrPC to a ProteinaseK-resistant beta-sheet-rich insoluble form PrPSC and its accumulation have been considered to be central to the pathogenesis of the prion diseases (TSE). In a recent paper, Deleault et al have shown that specific endogenous RNA molecules can induce in vitro structural conversion of endogenous PrPC to PrPSC. Small highly structured synthetic RNAs can also induce this conversion process. However, recent in vivo results show that PrPSC is not directly involved in the prion pathogenesis. It is possible, however, that nucleic-acid-induced PrPSC associated with the inducer nucleic acid could be the components of the infectious agent.

DNA-stimulated assembly of oligomeric bacteriophage 434 repressor: evidence for cooperative binding by recruitment

Typical of many transcriptional regulatory proteins, the lambdoid bacteriophage repressors bind cooperatively to multiple sites on DNA. This cooperative binding is essential for establishment and maintenance of phage lysogeny. In the phage, two repressor homodimers, one bound at each of the adjacent operator sites, interact to form the tetramer that is necessary for the cooperative binding of the repressor. Bacteriophage 434 repressor does not form tetramers in the absence of DNA, and the mechanism by which the tetramer assembles on the two adjacent sites is unknown. Hence DNA binding may stimulate the repressor to form tetramers and formation of a repressor oligomer (> or = 3 monomers) on a single DNA sites may precede multisite binding. Consistent with these ideas, a complex containing three repressor molecules readily assembles on a single operator (O(R)1) site. Mutations that inhibit cooperative tetramer binding to the adjacent O(R)1 and O(R)2 sites also block formation of this complex. Together with other evidence, these findings show that the complex that forms on a single site assembles using the same interface as does the tetramer assembled on adjacent operator sites. Adding additional O(R)1 DNA dissociates the oligomeric repressor-DNA complexes into dimeric repressor-O(R)1 complexes. In contrast, adding O(R)2 to these complexes results in the formation of a repressor oligomer containing an O(R)2 and an O(R)1 site. The observation that a repressor oligomer bound to two O(R)1 sites is less stable than the one formed between repressor dimers bound to O(R)1 and O(R)2 implies that DNA allosterically influences the structure of the 434 repressor. Together these findings suggest that an O(R)1-bound repressor may cooperatively help repressor bind to O(R)2 by recruiting an additional repressor molecule from solution that subsequently occupies O(R)2.

RAG1-DNA binding in V(D)J recombination. Specificity and DNA-induced conformational changes revealed by fluorescence and CD spectroscopy

The RAG1 and RAG2 proteins together constitute the nuclease that initiates the assembly of immunoglobulin and T cell receptor genes in a reaction known as V(D)J recombination. RAG1 plays a central role in recognition of the recombination signal sequence (RSS) by the RAG1/2 complex. To investigate the parameters governing the RAG1-RSS interaction, the murine core RAG1 protein (amino acids 377-1008) fused to a short Strep tag has been purified to homogeneity from bacteria. The Strep-RAG1 (StrRAG1) protein exists as a dimer at a wide range of protein concentrations (25-500 nM) in the absence of DNA and binds with reasonably high affinity and specificity (apparent K(D) = 41 nM) to the RSS. Both electrophoretic mobility shift assays and polarization anisotropy experiments indicate that only a single StrRAG1-DNA species exists in solution. Anisotropy decay measured by frequency domain spectroscopy suggests that the complex contains a dimer of StrRAG1 bound to a single DNA molecule. Using measurements of protein intrinsic fluorescence and circular dichroism, we demonstrate that StrRAG1 undergoes a major conformational change upon binding the RSS. Steady-state fluorescence and acrylamide quenching studies reveal that this conformational change is associated with a repositioning of intrinsic protein fluorophores from a hydrophobic to a solvent-exposed environment. RSS-induced conformational changes of StrRAG1 may influence the interaction of RAG1 with RAG2 and synaptic complex formation.

DNA-induced partial unfolding of prion protein leads to its polymerisation to amyloid

The full-length mouse recombinant prion protein (23-231 amino acid residues) contains all of its structural elements viz. three alpha-helices and a short two-stranded antiparallel beta-sheet in its C-terminal fragment comprising 121-231 amino acid residues. The incubated mixture of this prion protein fragment and nucleic acid results in the formation of amyloid fibres evidenced from electron microscopy, birefringence and fluorescence of the fibre bound Congo Red and Thioflavin T dyes, respectively. The secondary structure of the amyloid formed in nucleic acid solution is similar to the in vivo isolated prion protein 27-30 amyloid but unlike in it, a hydrophobic milieu is absent in the 121-231 amyloid. Thermal denaturation study demonstrates a partial unfolding of the protein fragment in nucleic acid solution. We propose that nucleic acid catalyses unfolding of prion protein helix 1 followed by a nucleation-dependent polymerisation of the protein to amyloid.

Mechanistic aspects of DnaA-RepA interaction as revealed by yeast forward and reverse two-hybrid analysis

Using yeast forward and reverse two-hybrid analysis and biochemical techniques, we present novel and definitive in vivo and in vitro evidence that both the N-terminal domain I and C-terminal domain IV of the host-encoded DnaA initiator protein of Escherichia coli interact physically with plasmid-encoded RepA initiator of pSC101. The N-terminal, but not the C-terminal, region of RepA interacted with DnaA in vitro. These protein-protein interactions are critical for two very early steps of replication initiation, namely origin unwinding and helicase loading. Neither domain I nor IV of DnaA could individually collaborate with RepA to promote pSC101 replication. However, when the two domains are co-expressed within a common cell milieu and allowed to associate non-covalently with each other via a pair of leucine zippers, replication of the plasmid was supported in vivo. Thus, the result shows that physical tethering, either non-covalent or covalent, of domain I and IV of DnaA and interaction of both domains with RepA, are critical for replication initiation. The results also provide the molecular basis for a novel, potential, replication-based bacterial two-hybrid system.

Bacteriophage lambda: alive and well and still doing its thing

The lambda (lambda) family of bacteriophages continues to provide significant insights into the understanding of basic biological processes, as well as useful technological innovations. Areas in which recent advances have occurred include transcription elongation, repressor interactions, genomics and post-transcriptional regulation. The homologous lambda recombination functions have been exploited as an efficient in vivo recombinant engineering system for functional genomic studies. The virulence of some pathogenic strains of Escherichia coli is enhanced by the expression of Shiga toxin (stx) genes encoded on a resident lambdoid prophage. Recent work suggests that the phage regulatory network may be a significant contributor to toxin production and release by these pathogenic E. coli.

Crystal structure of the iron-dependent regulator from Mycobacterium tuberculosis at 2.0-A resolution reveals the Src homology domain 3-like fold and metal binding function of the third domain

Iron-dependent regulators are primary transcriptional regulators of virulence factors and iron scavenging systems that are important for infection by several bacterial pathogens. Here we present the 2.0-A crystal structure of the wild type iron-dependent regulator from Mycobacterium tuberculosis in its fully active holorepressor conformation. Clear, unbiased electron density for the Src homology domain 3-like third domain, which is often invisible in structures of iron-dependent regulators, was revealed by density modification and averaging. This domain is one of the rare examples of Src homology domain 3-like folds in bacterial proteins, and, in addition, displays a metal binding function by contributing two ligands, one Glu and one Gln, to the pentacoordinated cobalt atom at metal site 1. Both metal sites are fully occupied, and tightly bound water molecules at metal site 1 ("Water 1") and metal site 2 ("Water 2") are identified unambiguously. The main chain carbonyl of Leu4 makes an indirect interaction with the cobalt atom at metal site 2 via Water 2, and the adjacent residue, Val5, forms a rare gamma turn. Residues 1-3 are well ordered and make numerous interactions. These ordered solvent molecules and the conformation and interactions of the N-terminal pentapeptide thus might be important in metal-dependent activation.

Thermostable and site-specific DNA binding of the gene product ORF56 from the Sulfolobus islandicus plasmid pRN1, a putative archael plasmid copy control protein

There is still a lack of information on the specific characteristics of DNA-binding proteins from hyperthermophiles. Here we report on the product of the gene orf56 from plasmid pRN1 of the acidophilic and thermophilic archaeon Sulfolobus islandicus. orf56 has not been characterised yet but low sequence similarily to several eubacterial plasmid-encoded genes suggests that this 6.5 kDa protein is a sequence-specific DNA-binding protein. The DNA-binding properties of ORF56, expressed in Escherichia coli, have been investigated by EMSA experiments and by fluorescence anisotropy measurements. Recombinant ORF56 binds to double-stranded DNA, specifically to an inverted repeat located within the promoter of orf56. Binding to this site could down-regulate transcription of the orf56 gene and also of the overlapping orf904 gene, encoding the putative initiator protein of plasmid replication. By gel filtration and chemical crosslinking we have shown that ORF56 is a dimeric protein. Stoichiometric fluorescence anisotropy titrations further indicate that ORF56 binds as a tetramer to the inverted repeat of its target binding site. CD spectroscopy points to a significant increase in ordered secondary structure of ORF56 upon binding DNA. ORF56 binds without apparent cooperativity to its target DNA with a dissociation constant in the nanomolar range. Quantitative analysis of binding isotherms performed at various salt concentrations and at different temperatures indicates that approximately seven ions are released upon complex formation and that complex formation is accompanied by a change in heat capacity of -6.2 kJ/mol. Furthermore, recombinant ORF56 proved to be highly thermostable and is able to bind DNA up to 85 degrees C.