Control of transcriptional pausing by biased thermal fluctuations on repetitive genomic sequences

Repetitive DNA symmetry elements negatively regulate gene expression in embryonic stem cells

Transcription factor (TF) binding to genomic DNA elements constitutes one of the key mechanisms that regulates gene expression program in cells. Both consensus and nonconsensus DNA sequence elements influence the recognition specificity of TFs. Based on the analysis of experimentally determined c-Myc binding preferences to genomic DNA, here we statistically predict that certain repetitive, nonconsensus DNA symmetry elements can relatively reduce TF-DNA binding preferences. This is in contrast to a different set of repetitive, nonconsensus symmetry elements that can increase the strength of TF-DNA binding. Using c-Myc enhancer reporter system containing consensus motif flanked by nonconsensus sequences in embryonic stem cells, we directly demonstrate that the enrichment in such negatively regulating repetitive symmetry elements is sufficient to reduce the gene expression level compared with native genomic sequences. Negatively regulating repetitive symmetry elements around consensus c-Myc motif and DNA sequences containing consensus c-Myc motif flanked by entirely randomized sequences show similar expression baseline. A possible explanation for this observation is that rather than complete repression, negatively regulating repetitive symmetry elements play a regulatory role in fine-tuning the reduction of gene expression, most probably by binding TFs other than c-Myc.

Gre factors prevent thermal and mechanical stresses induced by terahertz irradiation during transcription

During transcription in cells, the transcription complex consisting of RNA polymerase, DNA and nascent RNA is exposed to fluctuating temperature and pressure. However, little is known about the mechanism of transcriptional homeostasis under fluctuating physical parameters. In this study, we generated these fluctuating parameters using pulsed local heating and acoustic waves in the reaction system of transcription by Escherichia coli RNA polymerase, using a terahertz free-electron laser. We demonstrated that transcription processes, including abortive initiation and elongation pausing, and the fidelity of elongation are significantly affected by the laser-based local perturbations. We also found that all these functional alternations in the transcription process are almost completely mitigated by the presence of Gre proteins. It is well known that Gre proteins enhance RNA cleavage of polymerase by binding to the pore structure termed secondary channel. Recently, the chaperone activities have also been proposed for Gre proteins, yet the details directly associated with transcription are largely unknown. Our finding indicates that Gre proteins are necessary for maintaining transcriptional homeostasis under thermal and mechanical stresses.

Control of Transcription Initiation by Biased Thermal Fluctuations on Repetitive Genomic Sequences

In the process of transcription initiation by RNA polymerase, promoter DNA sequences affect multiple reaction pathways determining the productivity of transcription. However, the question of how the molecular mechanism of transcription initiation depends on the sequence properties of promoter DNA remains poorly understood. Here, combining the statistical mechanical approach with high-throughput sequencing results, we characterize abortive transcription and pausing during transcription initiation by Escherichia coli RNA polymerase at a genome-wide level. Our results suggest that initially transcribed sequences, when enriched with thymine bases, contain the signal for inducing abortive transcription, whereas certain repetitive sequence elements embedded in promoter regions constitute the signal for inducing pausing. Both signals decrease the productivity of transcription initiation. Based on solution NMR and in vitro transcription measurements, we suggest that repetitive sequence elements within the promoter DNA modulate the nonlocal base pair stability of its double-stranded form. This stability profoundly influences the reaction coordinates of the productive initiation via pausing.

Transcription Factor Binding in Embryonic Stem Cells Is Constrained by DNA Sequence Repeat Symmetry

Transcription factor (TF) recognition is dictated by the underlying DNA motif sequence specific for each TF. Here, we reveal that DNA sequence repeat symmetry plays a central role in defining TF-DNA-binding preferences. In particular, we find that different TFs bind similar symmetry patterns in the context of different developmental layers. Most TFs possess dominant preferences for similar DNA repeat symmetry types. However, in some cases, preferences of specific TFs are changed during differentiation, suggesting the importance of information encoded outside of known motif regions. Histone modifications also exhibit strong preferences for similar DNA repeat symmetry patterns unique to each type of modification. Next, using an in vivo reporter assay, we show that gene expression in embryonic stem cells can be positively modulated by the presence of genomic and computationally designed DNA oligonucleotides containing identified nonconsensus-repetitive sequence elements. This supports the hypothesis that certain nonconsensus-repetitive patterns possess a functional ability to regulate gene expression. We also performed a solution NMR experiment to probe the stability of double-stranded DNA via imino proton resonances for several double-stranded DNA sequences characterized by different repetitive patterns. We suggest that such local stability might play a key role in determining TF-DNA binding preferences. Overall, our findings show that despite the enormous sequence complexity of the TF-DNA binding landscape in differentiating embryonic stem cells, this landscape can be quantitatively characterized in simple terms using the notion of DNA sequence repeat symmetry.

C, Da LT, Yu J. A Viral T7 RNA Polymerase Ratcheting Along DNA With Fidelity Control

RNA polymerase (RNAP) from bacteriophage T7 is a representative single-subunit viral RNAP that can transcribe with high promoter activities without assistances from transcription factors. We accordingly studied this small transcription machine computationally as a model system to understand underlying mechanisms of mechano-chemical coupling and fidelity control in the RNAP transcription elongation. Here we summarize our computational work from several recent publications to demonstrate first how T7 RNAP translocates via Brownian alike motions along DNA right after the catalytic product release. Then we show how the backward translocation motions are prevented at post-translocation upon successful nucleotide incorporation, which is also subject to stepwise nucleotide selection and acts as a pawl for "selective ratcheting". The structural dynamics and energetics features revealed from our atomistic molecular dynamics (MD) simulations and related analyses on the single-subunit T7 RNAP thus provided detailed and quantitative characterizations on the Brownian-ratchet working scenario of a prototypical transcription machine with sophisticated nucleotide selectivity for fidelity control. The presented mechanisms can be more or less general for structurally similar viral or mitochondrial RNAPs and some of DNA polymerases, or even for the RNAP engine of the more complicated transcription machinery in higher organisms.

Single-round isolation of diverse RNA aptamers from a random sequence pool

Aptamers are oligonucleotide ligands with specific binding affinity to target molecules. Generally, RNA aptamers are selected from an RNA pool with random sequences, using the technique termed SELEX, in which the target-binding RNA molecules are repeatedly isolated and exponentially amplified. Despite several advantages, SELEX often produces uncertain results during the iterative amplifications of the rare target-binding RNA molecules. Here, we develop a non-repeated, primer-less and target immobilization-free isolation method for generating RNA aptamers, which is robust to experimental noise. Uniquely, this method focuses on finding and removal of non-aptamer sequences from the RNA pool by RNase digestion leaving target-bound aptamer molecules, and thus is independent of aptamer types. The undigested RNA sequences remaining are so few in number that they must be mixed with a large excess of a known sequence for further manipulations and this sequence is then removed by restriction digestion followed by high-throughput sequencing analysis to identify aptamers. Using this method, we generated multiple RNA aptamers targeting α-thrombin and TGFβ1 proteins, independently. This method potentially generates thousands of sequences as aptamer candidates, which may enable us to predict a common average sequence or structural property of these aptamers that is different from input RNA.

Transcription pausing: biological significance of thermal fluctuations biased by repetitive genomic sequences

Transcription of DNA by RNA polymerase (RNAP) takes place in a cell environment dominated by thermal fluctuations. How are transcription reactions including initiation, elongation, and termination on genomic DNA so well-controlled during such fluctuations? A recent statistical mechanical approach using high-throughput sequencing data reveals that repetitive DNA sequence elements embedded into a genomic sequence provide the key mechanism to functionally bias the fluctuations of transcription elongation complexes. In particular, during elongation pausing, such repetitive sequence elements can increase the magnitude of one-dimensional diffusion of the RNAP enzyme on the DNA upstream of the pausing site, generating a large variation in the dwell times of RNAP pausing under the control of these genomic signals.