A thermodynamic model of the cooperative interaction between the archaeal transcription factor Ss-LrpB and its tripartite operator DNA

Predictive modeling reveals that higher-order cooperativity drives transcriptional repression in a synthetic developmental enhancer

A challenge in quantitative biology is to predict output patterns of gene expression from knowledge of input transcription factor patterns and from the arrangement of binding sites for these transcription factors on regulatory DNA. We tested whether widespread thermodynamic models could be used to infer parameters describing simple regulatory architectures that inform parameter-free predictions of more complex enhancers in the context of transcriptional repression by Runt in the early fruit fly embryo. By modulating the number and placement of Runt binding sites within an enhancer, and quantifying the resulting transcriptional activity using live imaging, we discovered that thermodynamic models call for higher-order cooperativity between multiple molecular players. This higher-order cooperativity captures the combinatorial complexity underlying eukaryotic transcriptional regulation and cannot be determined from simpler regulatory architectures, highlighting the challenges in reaching a predictive understanding of transcriptional regulation in eukaryotes and calling for approaches that quantitatively dissect their molecular nature.

Separation and Characterization of Protein-DNA Complexes by EMSA and In-Gel Footprinting

In-gel footprinting enables the precise identification of protein binding sites on the DNA after separation of free and protein-bound DNA molecules by gel electrophoresis in native conditions and subsequent digestion by the nuclease activity of the 1,10-phenanthroline-copper ion [(OP)₂-Cu⁺] within the gel matrix. Hence, the technique combines the resolving power of protein-DNA complexes in the electrophoretic mobility shift assay (EMSA) with the precision of target site identification by chemical footprinting. This approach is particularly well suited to characterize distinct molecular assemblies in a mixture of protein-DNA complexes and to identify individual binding sites within composite operators, when the concentration-dependent occupation of binding sites, with a different affinity, results in the generation of complexes with a distinct stoichiometry and migration velocity in gel electrophoresis.

Allosteric conformational ensembles have unlimited capacity for integrating information

Integration of binding information by macromolecular entities is fundamental to cellular functionality. Recent work has shown that such integration cannot be explained by pairwise cooperativities, in which binding is modulated by binding at another site. Higher-order cooperativities (HOCs), in which binding is collectively modulated by multiple other binding events, appear to be necessary but an appropriate mechanism has been lacking. We show here that HOCs arise through allostery, in which effective cooperativity emerges indirectly from an ensemble of dynamically interchanging conformations. Conformational ensembles play important roles in many cellular processes but their integrative capabilities remain poorly understood. We show that sufficiently complex ensembles can implement any form of information integration achievable without energy expenditure, including all patterns of HOCs. Our results provide a rigorous biophysical foundation for analysing the integration of binding information through allostery. We discuss the implications for eukaryotic gene regulation, where complex conformational dynamics accompanies widespread information integration.

Gene Regulation in and out of Equilibrium

Determining whether and how a gene is transcribed are two of the central processes of life. The conceptual basis for understanding such gene regulation arose from pioneering biophysical studies in eubacteria. However, eukaryotic genomes exhibit vastly greater complexity, which raises questions not addressed by this bacterial paradigm. First, how is information integrated from many widely separated binding sites to determine how a gene is transcribed? Second, does the presence of multiple energy-expending mechanisms, which are absent from eubacterial genomes, indicate that eukaryotes are capable of improved forms of genetic information processing? An updated biophysical foundation is needed to answer such questions. We describe the linear framework, a graph-based approach to Markov processes, and show that it can accommodate many previous studies in the field. Under the assumption of thermodynamic equilibrium, we introduce a language of higher-order cooperativities and show how it can rigorously quantify gene regulatory properties suggested by experiment. We point out that fundamental limits to information processing arise at thermodynamic equilibrium and can only be bypassed through energy expenditure. Finally, we outline some of the mathematical challenges that must be overcome to construct an improved biophysical understanding of gene regulation.

Non-monotonic auto-regulation in single gene circuits

We theoretically study the effects of non-monotonic response curves in genetic auto-regulation by exploring the possible dynamical behaviors for such systems. Our motivation is twofold: we aim at conceiving the simplest genetic circuits for synthetic biology and at understanding the natural auto-regulation of the LrpB protein of the Sulfolobus solfataricus archaeon which exhibits non-monotonicity. We analyzed three toy models, based on mass-action kinetics, with increasing complexity and sought for oscillations and (fast) bistable switching. We performed large parameter scans and sensitivity analyses, and quantified the quality of the oscillators and switches by computing relative volumes in parameter space reproducing the sought dynamical behavior. All single gene systems need finely tuned parameters in order to oscillate, but bistable switches are more robust against parameter changes. We expected non-monotonic switches to be faster than monotonic ones, however solutions combining both auto-activation and repression in the physiological range to obtain fast switches are scarce. Our analysis shows that the Ss-LrpB system can not provide a bistable switch and that robust oscillations are unlikely. Gillespie simulations suggest that the function of the natural Ss-LrpB system is sensing via a spiking behavior, which is in line with the fact that this protein has a metabolic regulatory function and binds to a ligand.

DNA-Interacting Characteristics of the Archaeal Rudiviral Protein SIRV2_Gp1

Whereas the infection cycles of many bacterial and eukaryotic viruses have been characterized in detail, those of archaeal viruses remain largely unexplored. Recently, studies on a few model archaeal viruses such as SIRV2 (Sulfolobus islandicus rod-shaped virus) have revealed an unusual lysis mechanism that involves the formation of pyramidal egress structures on the host cell surface. To expand understanding of the infection cycle of SIRV2, we aimed to functionally characterize gp1, which is a SIRV2 gene with unknown function. The SIRV2_Gp1 protein is highly expressed during early stages of infection and it is the only protein that is encoded twice on the viral genome. It harbours a helix-turn-helix motif and was therefore hypothesized to bind DNA. The DNA-binding behavior of SIRV2_Gp1 was characterized with electrophoretic mobility shift assays and atomic force microscopy. We provide evidence that the protein interacts with DNA and that it forms large aggregates, thereby causing extreme condensation of the DNA. Furthermore, the N-terminal domain of the protein mediates toxicity to the viral host Sulfolobus. Our findings may lead to biotechnological applications, such as the development of a toxic peptide for the containment of pathogenic bacteria, and add to our understanding of the Rudiviral infection cycle.

The genome-scale DNA-binding profile of BarR, a β-alanine responsive transcription factor in the archaeon Sulfolobus acidocaldarius

Background

The Leucine-responsive Regulatory Protein (Lrp) family is a widespread family of regulatory transcription factors in prokaryotes. BarR is an Lrp-like transcription factor in the model archaeon Sulfolobus acidocaldarius that activates the expression of a β-alanine aminotransferase gene, which is involved in β-alanine degradation. In contrast to classical Lrp-like transcription factors, BarR is not responsive to any of the α-amino acids but interacts specifically with β-alanine. Besides the juxtaposed β-alanine aminotransferase gene, other regulatory targets of BarR have not yet been identified although β-alanine is the precursor of coenzyme A and thus an important central metabolite. The aim of this study is to extend the knowledge of the DNA-binding characteristics of BarR and of its corresponding regulon from a local to a genome-wide perspective.

Results

We characterized the genome-wide binding profile of BarR using chromatin immunoprecipation combined with high-throughput sequencing (ChIP-seq). This revealed 21 genomic binding loci. High-enrichment binding regions were validated to interact with purified BarR protein in vitro using electrophoretic mobility shift assays and almost all targets were also shown to harbour a conserved semi-palindromic binding motif. Only a small subset of enriched genomic sites are located in intergenic regions at a relative short distance to a promoter, and qRT-PCR analysis demonstrated that only one additional operon is under activation of BarR, namely the glutamine synthase operon. The latter is also a target of other Lrp-like transcription factors. Detailed inspection of the BarR ChIP-seq profile at the β-alanine aminotransferase promoter region in combination with binding motif predictions indicate that the operator structure is more complicated than previously anticipated, consisting of multiple (major and auxiliary) operators.

Conclusions

BarR has a limited regulon, and includes also glutamine synthase genes besides the previously characterized β-alanine aminotransferase. Regulation of glutamine synthase is suggestive of a link between β-alanine and α-amino acid metabolism in S. acidocaldarius. Furthermore, this work reveals that the BarR regulon overlaps with that of other Lrp-like regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2890-0) contains supplementary material, which is available to authorized users.

Regulation Mechanism of the ald Gene Encoding Alanine Dehydrogenase in Mycobacterium smegmatis and Mycobacterium tuberculosis by the Lrp/AsnC Family Regulator AldR

In the presence of alanine, AldR, which belongs to the Lrp/AsnC family of transcriptional regulators and regulates ald encoding alanine dehydrogenase in Mycobacterium smegmatis, changes its quaternary structure from a homodimer to an octamer with an open-ring conformation. Four AldR-binding sites (O2, O1, O4, and O3) with a consensus sequence of GA/T-N2-NWW/WWN-N2-A/TC were identified upstream of the M. smegmatis ald gene by means of DNase I footprinting analysis. O2, O1, and O4 are required for the induction of ald expression by alanine, while O3 is directly involved in the repression of ald expression. In addition to O3, both O1 and O4 are also necessary for full repression of ald expression in the absence of alanine, due to cooperative binding of AldR dimers to O1, O4, and O3. Binding of a molecule of the AldR octamer to the ald control region was demonstrated to require two AldR-binding sites separated by three helical turns between their centers and one additional binding site that is in phase with the two AldR-binding sites. The cooperative binding of AldR dimers to DNA requires three AldR-binding sites that are aligned with a periodicity of three helical turns. The aldR gene is negatively autoregulated independently of alanine. Comparative analysis of ald expression of M. smegmatis and Mycobacterium tuberculosis in conjunction with sequence analysis of both ald control regions led us to suggest that the expression of the ald genes in both mycobacterial species is regulated by the same mechanism.

IMPORTANCE

In mycobacteria, alanine dehydrogenase (Ald) is the enzyme required both to utilize alanine as a nitrogen source and to grow under hypoxic conditions by maintaining the redox state of the NADH/NAD(+) pool. Expression of the ald gene was reported to be regulated by the AldR regulator that belongs to the Lrp/AsnC (feast/famine) family, but the underlying mechanism was unknown. This study revealed the regulation mechanism of ald in Mycobacterium smegmatis and Mycobacterium tuberculosis. Furthermore, a generalized arrangement pattern of cis-acting regulatory sites for Lrp/AsnC (feast/famine) family regulators is suggested in this study.

BarR, an Lrp-type transcription factor in Sulfolobus acidocaldarius, regulates an aminotransferase gene in a β-alanine responsive manner

In archaea, nothing is known about the β-alanine degradation pathway or its regulation. In this work, we identify and characterize BarR, a novel Lrp-like transcription factor and the first one that has a non-proteinogenic amino acid ligand. BarR is conserved in Sulfolobus acidocaldarius and Sulfolobus tokodaii and is located in a divergent operon with a gene predicted to encode β-alanine aminotransferase. Deletion of barR resulted in a reduced exponential growth rate in the presence of β-alanine. Furthermore, qRT-PCR and promoter activity assays demonstrated that BarR activates the expression of the adjacent aminotransferase gene, but only upon β-alanine supplementation. In contrast, auto-activation proved to be β-alanine independent. Heterologously produced BarR is an octamer in solution and forms a single complex by interacting with multiple sites in the 170 bp long intergenic region separating the divergently transcribed genes. In vitro, DNA binding is specifically responsive to β-alanine and site-mutant analyses indicated that β-alanine directly interacts with the ligand-binding pocket. Altogether, this work contributes to the growing body of evidence that in archaea, Lrp-like transcription factors have physiological roles that go beyond the regulation of α-amino acid metabolism.

The 'Densitometric Image Analysis Software' and its application to determine stepwise equilibrium constants from electrophoretic mobility shift assays

Current software applications for densitometric analysis, such as ImageJ, QuantityOne (BioRad) and the Intelligent or Advanced Quantifier (Bio Image) do not allow to take the non-linearity of autoradiographic films into account during calibration. As a consequence, quantification of autoradiographs is often regarded as problematic, and phosphorimaging is the preferred alternative. However, the non-linear behaviour of autoradiographs can be described mathematically, so it can be accounted for. Therefore, the ‘Densitometric Image Analysis Software’ has been developed, which allows to quantify electrophoretic bands in autoradiographs, as well as in gels and phosphorimages, while providing optimized band selection support to the user. Moreover, the program can determine protein-DNA binding constants from Electrophoretic Mobility Shift Assays (EMSAs). For this purpose, the software calculates a chosen stepwise equilibrium constant for each migration lane within the EMSA, and estimates the errors due to non-uniformity of the background noise, smear caused by complex dissociation or denaturation of double-stranded DNA, and technical errors such as pipetting inaccuracies. Thereby, the program helps the user to optimize experimental parameters and to choose the best lanes for estimating an average equilibrium constant. This process can reduce the inaccuracy of equilibrium constants from the usual factor of 2 to about 20%, which is particularly useful when determining position weight matrices and cooperative binding constants to predict genomic binding sites. The MATLAB source code, platform-dependent software and installation instructions are available via the website http://micr.vub.ac.be.

The genome-wide binding profile of the Sulfolobus solfataricus transcription factor Ss-LrpB shows binding events beyond direct transcription regulation

Background

Gene regulatory processes are largely resulting from binding of transcription factors to specific genomic targets. Leucine-responsive Regulatory Protein (Lrp) is a prevalent transcription factor family in prokaryotes, however, little information is available on biological functions of these proteins in archaea. Here, we study genome-wide binding of the Lrp-like transcription factor Ss-LrpB from Sulfolobus solfataricus.

Results

Chromatin immunoprecipitation in combination with DNA microarray analysis (ChIP-chip) has revealed that Ss-LrpB interacts with 36 additional loci besides the four previously identified local targets. Only a subset of the newly identified binding targets, concentrated in a highly variable IS-dense genomic region, is also bound in vitro by pure Ss-LrpB. There is no clear relationship between the in vitro measured DNA-binding specificity of Ss-LrpB and the in vivo association suggesting a limited permissivity of the crenarchaeal chromatin for transcription factor binding. Of 37 identified binding regions, 29 are co-bound by LysM, another Lrp-like transcription factor in S. solfataricus. Comparative gene expression analysis in an Ss-lrpB mutant strain shows no significant Ss-LrpB-mediated regulation for most targeted genes, with exception of the CRISPR B cluster, which is activated by Ss-LrpB through binding to a specific motif in the leader region.

Conclusions

The genome-wide binding profile presented here implies that Ss-LrpB is associated at additional genomic binding sites besides the local gene targets, but acts as a specific transcription regulator in the tested growth conditions. Moreover, we have provided evidence that two Lrp-like transcription factors in S. solfataricus, Ss-LrpB and LysM, interact in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-14-828) contains supplementary material, which is available to authorized users.