Biology subjects

Fei, J.

Publications and source records attributed to Fei, J..

26 records · Page 2Linked to original sources

bioRxiv2022

Skeletal muscle lineage is dispensable for appendage regeneration in axolotl

Regeneration of a complex appendage structure such as limb requires hierarchical coordination of multiple types of tissues. Muscles, as one of the major cell masses in limbs, have been reported recently to be critical to guide other tissue regeneration in planaria, but its function and relationship to other cells in vertebrate complex regeneration have been unclear. Here, we use Pax7 mutant axolotls, in which the limb muscle is developmentally lost, to investigate limb regeneration in the absence of skeletal muscle. We find that the pattern of regenerated limbs is normal in Pax7 mutants compared to the controls. Lack of muscles do not affect the proliferation of fibroblasts, another major population in limbs. Furthermore, using single cell RNA-sequencing, we demonstrate that the cell type composition in completely regenerated limbs in Pax7 mutants is similar to that in the controls, except the lack of cell types in muscle lineage. Our study reveals skeletal muscle is not required for the guidance of complex tissue regeneration in axolotls, and provides new views of the tissue hierarchy in vertebrate appendage regeneration.

cell biology↗

bioRxiv2020

Kinetic model of small RNA-mediated regulation suggests that a small RNA can regulate co-transcriptionally

Small RNAs (sRNAs) are important regulators of gene expression in bacteria, particularly during stress responses. Many genetically and biochemically well characterized sRNAs regulate gene expression post-transcriptionally, by affecting translation and degradation of the target mRNA after they bind to their targets through base pairing. However, how regulation at each of these levels quantitatively contributes to the overall efficacy of sRNA-mediated regulation is not well understood. Here we present a general approach combining imaging and mathematical modeling to determine kinetic parameters at different levels of sRNA-mediated gene regulation. Unexpectedly, our data reveal that certain previously characterized sRNAs are able to regulate some targets co-transcriptionally, rather strictly post-transcriptionally, and suggest that sRNA-mediated regulation can occur early in the mRNAs lifetime, perhaps as soon as the sRNA binding site is transcribed. In addition, our data suggest several important kinetic steps that may determine the efficiency and differential regulation of multiple mRNA targets by an sRNA. Particularly, binding of sRNA to the target mRNA is likely the rate-limiting step and may dictate the regulation hierarchy observed within an sRNA regulon.

biophysics↗

bioRxiv2020

Interaction Landscape of K29-Linked Ubiquitin Signaling Revealed by a Linkage-Specific Synthetic Antigen-Binding Fragment

Protein ubiquitination shows remarkable topological and functional diversity through the polymerization of ubiquitin via different linkages. Deciphering the cellular ubiquitin code is of central importance to understand the physiology of the cell. Among the eight possible linkages, K29-linked polyubiquitin is a relatively abundant type of polyubiquitin in both human and yeast cells. However, our understanding of its function is rather limited due to the lack of specific binders as tools to detect K29-linked polyubiquitin. In this study, we screened and characterized a synthetic antigen-binding fragment, termed sAB-K29, that can specifically recognize K29-linked polyubiquitin using chemically synthesized K29-linked diubiquitin. We further determined the crystal structure of this fragment bound to the K29-linked diubiquitin, which revealed the molecular basis of specificity. Using sAB-K29 as a tool, we uncovered that K29-linked ubiquitination is involved in different kinds of cellular proteotoxic stress response as well as cell cycle regulation. In particular, we showed that K29-linked ubiquitination is enriched in the midbody and downregulation of the K29-linked ubiquitination signal arrests cells in G1/S phase.

cell biology↗

bioRxiv2020

Effects of individual base-pairs on in vivo target search and destruction kinetics of small RNA

Base-pairing interactions mediate intermolecular target recognition in many biological systems and applications, including DNA repair, CRISPR, microRNA, small RNA (sRNA) and antisense oligo therapies. Even a single base-pair mismatch can cause a substantial difference in biological activity but presently we do not yet know how the target search kinetics in vivo are influenced by single nucleotide level changes. Here, we used high-throughput sequencing to identify functionally relevant single point mutants of the bacterial sRNA, SgrS, and quantitative super-resolution microscopy to probe the mutational impact on the regulation of its primary target, ptsG mRNA. Our super-resolution imaging and analysis platform allowed us to further dissect mutational effects on SgrS lifetimes, and even subtle changes in the in vivo rates of target association, kon, and dissociation, koff. Mutations that disrupt binding of a chaperone protein, Hfq, and are distal to the mRNA annealing region still decreased kon and increased koff, providing an in vivo demonstration that Hfq directly facilitates sRNA-mRNA annealing. Single base-pair mismatches in the annealing region reduced kon by 24-31% and increased koff by 14-25%, extending the time it takes to find and destroy the target mRNA by about a third, depending on whether an AU or GC base-pair is disrupted. The effects of disrupting contiguous base-pairing are much more modest than that expected from thermodynamics, suggesting that Hfq also buffers base-pair disruptions.

molecular biology↗

bioRxiv2020

Deoxyribozyme-based Method for Site-specific Absolute Quantification of N6-methyladenosine Modification Fraction

N6-methyladenosine (m6A) is the most prevalent modified base in eukaryotic messenger RNA (mRNA) and long noncoding RNA (lncRNA). Although candidate sites for m6A modification are identified at the transcriptomic level, site-specific quantification methods for m6A modifications are still limited. Herein, we present a facile method implementing deoxyribozyme that preferentially cleaves the unmodified RNA. We leverage reverse transcription and real-time quantitative PCR along with key control experiments to quantify the absolute methylation fraction of specific m6A sites. We validate the accuracy of the method using synthetic RNA with controlled methylation fraction and apply our method on several endogenous sites that were previously identified in sequencing-based studies. This method provides a time and cost-effective approach for absolute quantification of the m6A fraction at specific loci, expanding the current toolkit for studying RNA modifications.

biochemistry↗

bioRxiv2020

Dynamic interactions between the RNA chaperone Hfq, small regulatory RNAs and mRNAs in live bacterial cells

RNA binding proteins play myriad roles in controlling and regulating RNAs and RNA-mediated functions, often through simultaneous binding to other cellular factors. In bacteria, the RNA chaperone Hfq modulates post-transcriptional gene regulation. Absence of Hfq leads to the loss of fitness and compromises the virulence of bacterial pathogens. Using live-cell super-resolution imaging, we are able to distinguish Hfq binding to different sizes of cellular RNAs. We demonstrate that under normal growth conditions, Hfq exhibits widespread mRNA binding activity. Particularly, the distal face of Hfq contributes mostly to the mRNA binding in vivo. In addition, binding of Hfq to these mRNAs can recruit RNase E to promote turnover of these mRNAs in an sRNA-independent manner, providing one mechanism to release Hfq from the pre-bound mRNAs. Finally, our data indicate that sRNAs, once expressed, can either co-occupy Hfq with the mRNA or displace the mRNA from Hfq, suggesting mechanisms through which sRNAs rapidly access Hfq to induce sRNA-mediated gene regulation. Our data collectively demonstrate that Hfq dynamically changes its interactions with different RNAs in response to changes in cellular conditions.

biophysics↗

bioRxiv2018

Two-step binding kinetics of tRNAGly by the glyQS T-box riboswitch and its regulation by T-box structural elements

T-box riboswitches are cis-regulatory RNA elements that regulate mRNAs encoding for aminoacyl tRNA synthetases or proteins involved in amino acid biosynthesis and transport. Rather than using small molecules as their ligands, as do most riboswitches, T-box riboswitches uniquely bind tRNA and sense their aminoacylated state. Whereas the anticodon and elbow regions of the tRNA interact with Stem I, located in the 5 portion of the T-box, sensing of the aminoacylation state involves direct binding of the NCCA sequence at the tRNA 3 end to the anti-terminator sequence located in the 3 portion of the T-box. However, the kinetic trajectory that describes how each of these interactions are established temporally during tRNA binding remains unclear. Using singlemolecule fluorescence resonance energy transfer (smFRET), we demonstrate that tRNA binds to the riboswitch in a two-step process, first with anticodon recognition followed by NCCA binding, with the second step accompanied by an inward motion of the 3 portion of the T-box riboswitch relative to Stem I. By using site-specific mutants, we further show that the T-loop region of the T-box significantly contributes to the first binding step, and that the K-turn region of the T-box influences both binding steps, but with a more dramatic effect on the second binding step. Our results set up a kinetic framework describing tRNA binding by T-box riboswitches and highlight the important roles of several T-box structural elements in regulating each binding step.\n\nSIGNIFICANCEBacteria commonly use riboswitches, cis-regulatory RNA elements, to regulate the transcription or translation of the mRNAs upon sensing signals. Unlike small molecule binding riboswitches, T-box riboswitches bind tRNA and sense their aminoacylated state. T-box modular structural elements that recognize different parts of a tRNA have been identified, however, how each of these interactions is established temporally during tRNA binding remains unclear. Our study reveals that tRNA binds to the riboswitch in a two-step mechanism, with anticodon recognition first, followed by binding to the NCCA sequence at the 3 end of the tRNA with concomitant conformational changes in the T-box. Our results also highlight the importance of the modular structural elements of the T-box in each of the binding steps.

biophysics↗

bioRxiv2017

Mechanisms of improved specificity of engineered Cas9s revealed by single molecule analysis

In microbes, CRISPR-Cas systems provide adaptive immunity against invading genetic elements. Cas9 in complex with a guide-RNA targets complementary DNA for cleavage and has been repurposed for wide-ranging biological applications. New Cas9s have been engineered (eCas9 and Cas9-HF1) to improve specificity, but how they help reduce off-target cleavage is not known. Here, we developed single molecule DNA unwinding assay to show that sequence mismatches affect cleavage reactions through rebalancing the internal unwinding/rewinding equilibrium. Increasing PAM-distal mismatches facilitate rewinding, and the associated cleavage impairment shows that cleavage proceeds from the unwound state. Engineered Cas9s depopulate the unwound state more readily upon mismatch detection. Intrinsic cleavage rate is much lower for engineered Cas9s, preventing cleavage from transiently unwound off-targets. DNA interrogation experiments showed that engineered Cas9s require about one additional base pair match for stable binding, freeing them from sites that would otherwise sequester them. Therefore, engineered Cas9s achieve their improved specificity (1) by inhibiting stable DNA binding to partially matching sequences, (2) by making DNA unwinding more sensitive to mismatches, and (3) by slowing down intrinsic cleavage reaction.

biophysics↗