Lipid oxidation in food emulsions is mediated by emulsifiers in the water phase and at the oil-water interface. To unravel the physico-chemical mechanisms and to obtain local lipid and protein oxidation rates, we used confocal laser scanning microscopy (CLSM), thereby monitoring changes in both the fluorescence emission of a lipophilic dye BODIPY 665/676 and protein auto-fluorescence. Our data show that the removal of lipid-soluble antioxidants from mayonnaises promotes lipid oxidation within oil droplets as well as protein oxidation at the oil-water interface. Furthermore, we demonstrate that ascorbic acid acts as either a lipid antioxidant or pro-oxidant depending on the presence of lipid-soluble antioxidants. The effects of antioxidant formulation on local lipid and protein oxidation rates were all statistically significant (p < 0.0001). The observed protein oxidation at the oil-water interface was spatially heterogeneous, which is in line with the heterogeneous distribution of lipoprotein granules from the egg yolk used for emulsification. The impact of the droplet size on local lipid and protein oxidation rates was significant (p < 0.0001) but minor compared to the effects of ascorbic acid addition and lipid-soluble antioxidant depletion. The presented results demonstrate that CLSM can be applied for unraveling the roles of colloidal structure and transport in mediating lipid oxidation in complex food emulsions.
Single-particle tracking is an important technique in the life sciences to understand the kinetics of biomolecules. The analysis of apparent diffusion coefficients in vivo, for example, enables researchers to determine whether biomolecules are moving alone, as part of a larger complex, or are bound to large cellular components such as the membrane or chromosomal DNA. A remaining challenge has been to retrieve quantitative kinetic models, especially for molecules that rapidly switch between different diffusional states. Here, we present analytical diffusion distribution analysis (anaDDA), a framework that allows for extracting transition rates from distributions of apparent diffusion coefficients calculated from short trajectories that feature less than 10 localizations per track. Under the assumption that the system is Markovian and diffusion is purely Brownian, we show that theoretically predicted distributions accurately match simulated distributions and that anaDDA outperforms existing methods to retrieve kinetics, especially in the fast regime of 0.1–10 transitions per imaging frame. AnaDDA does account for the effects of confinement and tracking window boundaries. Furthermore, we added the option to perform global fitting of data acquired at different frame times to allow complex models with multiple states to be fitted confidently. Previously, we have started to develop anaDDA to investigate the target search of CRISPR-Cas complexes. In this work, we have optimized the algorithms and reanalyzed experimental data of DNA polymerase I diffusing in live Escherichia coli. We found that long-lived DNA interaction by DNA polymerase are more abundant upon DNA damage, suggesting roles in DNA repair. We further revealed and quantified fast DNA probing interactions that last shorter than 10 ms. AnaDDA pushes the boundaries of the timescale of interactions that can be probed with single-particle tracking and is a mathematically rigorous framework that can be further expanded to extract detailed information about the behavior of biomolecules in living cells.
The hormone auxin controls many aspects of the plant life cycle by regulating the expression of thousands of genes. The transcriptional output of the nuclear auxin signaling pathway is determined by the activity of AUXIN RESPONSE transcription FACTORs (ARFs), through their binding to cis-regulatory elements in auxin-responsive genes. Crystal structures, in vitro, and heterologous studies have fueled a model in which ARF dimers bind with high affinity to distinctly spaced repeats of canonical AuxRE motifs. However, the relevance of this “caliper” model, and the mechanisms underlying the binding affinities in vivo, have remained elusive. Here we biochemically and functionally interrogate modes of ARF–DNA interaction. We show that a single additional hydrogen bond in Arabidopsis ARF1 confers high-affinity binding to individual DNA sites. We demonstrate the importance of AuxRE cooperativity within repeats in the Arabidopsis TMO5 and IAA11 promoters in vivo. Meta-analysis of transcriptomes further reveals strong genome-wide association of auxin response with both inverted (IR) and direct (DR) AuxRE repeats, which we experimentally validated. The association of these elements with auxin-induced up-regulation (DR and IR) or down-regulation (IR) was correlated with differential binding affinities of A-class and B-class ARFs, respectively, suggesting a mechanistic basis for the distinct activity of these repeats. Our results support the relevance of high-affinity binding of ARF transcription factors to uniquely spaced DNA elements in vivo, and suggest that differential binding affinities of ARF subfamilies underlie diversity in cis-element function.
Auxin controls numerous growth processes in land plants through a gene expression system that modulates ARF tran-scription factor activity. Gene duplications in families encoding auxin response components have generated tremendous complexity in most land plants, and neofunctionalization enabled various unique response outputs during development. However, it is unclear what fundamental biochemical principles underlie this complex response system. By studying the minimal system in Marchantia polymorpha, we derive an intuitive and simple model where a single auxin-dependent A-ARF activates gene expression. It is antagonized by an auxin-independent B-ARF that represses common target genes. The expression patterns of both ARF proteins define developmental zones where auxin response is permitted, quantitatively tuned or prevented. This fundamental design probably represents the ancestral system and formed the basis for inflated, complex systems.
C. Fijen, M. Kronenberg, R. Kaup, M. Fontana, J. Towle-Weicksel, J. Sweasy, J. Hohlbein, Journal of Biological Chemistry, 295, 9012–20, 2020, [link], previous bioRxiv preprint: [link]
Hydrogels made of the polysaccharide κ-carrageenan are widely used in the food and personal care industry as thickeners or gelling agents. These hydrogels feature dense regions embedded in a coarser bulk network, but the characteristic size and behavior of these regions has remained elusive. Here, we use single-particle-tracking fluorescence microscopy (sptFM) to quantitatively describe κ-carrageenan gels. Infusing fluorescent probes into fully gelated κ-carrageenan hydrogels resulted in two distinct diffusional behaviors. Obstructed self-diffusion of the probes revealed that the coarse network consists of κ-carrageenan strands with a typical diameter of 3.2 ± 0.3 nm leading to a nanoprobe diffusion coefficient of ~1-5∙10^-12 m2/s. In the dense network regions, we found a fraction with a largely decreased diffusion coefficient of ~1∙10^-13 m2/s. We also observed dynamic exchange between these states. The computation of spatial mobility maps from diffusional data indicated that the dense network regions have a characteristic diameter of ~1 µm and are itself mobile on the seconds-to-minutes timescale. sptFM provides an unprecedented view on spatiotemporal heterogeneity of hydrogel networks, which we believe bears general relevance for understanding transport and release of both low- and high molecular weight solutes.
CRISPR-Cas systems encode RNA-guided surveil-lance complexes to find and cleave invading DNA elements. While it is thought that invaders are neutralized minutes after cell entry, the mechanism andkinetics of target search and its impact on CRISPRprotection levels have remained unknown. Here, wevisualize individual Cascade complexes in a native type I CRISPR-Cas system. We uncover an exponential relation between Cascade copy number and CRISPR interference levels, pointing to a time-driven arms race between invader replication and target search, in which 20 Cascade complexes provide 50% protection. Driven by PAM-interacting subunitCas8e, Cascade spends half its search time rapidly probing DNA (30 ms) in the nucleoid. We further demonstrate that target DNA transcription and CRISPR arrays affect the integrity of Cascade and affect CRISPR interference. Our work establishes the mechanism of cellular DNA surveillance by Cascade that allows the timely detection of invading DNA in a crowded, DNA-packed environment.
47, 10788, 2019, [link]
, Nucleic Acid Research,
DNA-binding proteins utilise different recognition mechanisms to locate their DNA targets; some proteins recognise specific DNA sequences, while others interact with specific DNA structures. While sequence-specific DNA binding has been studied extensively, structure-specific recognition mechanisms remain unclear. Here, we study structure-specific DNA recognition by examining the structure and dynamics of DNA polymerase I Klenow Fragment (Pol) substrates both alone and in DNA–Pol complexes. Using a docking approach based on a network of 73 distances collected using single-molecule FRET, we determined a novel solution structure of the single-nucleotide-gapped DNA–Pol binary complex. The structure resembled existing crystal structures with regards to the downstream primer-template DNA substrate, and revealed a previously unobserved sharp bend (∼120°) in the DNA substrate; this pronounced bend was present in living cells. MD simulations and single-molecule assays also revealed that 4–5 nt of downstream gap-proximal DNA are unwound in the binary complex. Further, experiments and coarse-grained modelling showed the substrate alone frequently adopts bent conformations with 1–2 nt fraying around the gap, suggesting a mechanism wherein Pol recognises a pre-bent, partially-melted conformation of gapped DNA. We propose a general mechanism for substrate recognition by structure-specific enzymes driven by protein sensing of the conformational dynamics of their DNA substrates.
K.J.A. Martens, S. van Beljouw, S. van der Els, J.N.A. Vink, S. Baas, G.A. Vogelaar, S.J.J. Brouns, P. van Baarlen, M. Kleerebezem, J. Hohlbein, Nature Communications, 10, 3552, 2019, [link]
CRISPR-Cas9 is widely used in genomic editing, but the kinetics of target search and its relation to the cellular concentration of Cas9 have remained elusive. Effective target search requires constant screening of the protospacer adjacent motif (PAM) and a 30 ms upper limit for screening was recently found. To further quantify the rapid switching between DNA-bound and freely-diffusing states of dCas9, we developed an open-microscopy framework, the miCube, and introduce Monte-Carlo diffusion distribution analysis (MC-DDA). Our analysis reveals that dCas9 is screening PAMs 40% of the time in Gram-positive Lactoccous lactis, averaging 17 ± 4 ms per binding event. Using heterogeneous dCas9 expression, we determine the number of cellular target-containing plasmids and derive the copy number dependent Cas9 cleavage. Furthermore, we show that dCas9 is not irreversibly bound to target sites but can still interfere with plasmid replication. Taken together, our quantitative data facilitates further optimization of the CRISPR-Cas toolbox.
S.P.B. van Beljouw, S. van der Els, K.J. A. Martens, M. Kleerebezem, P.A. Bron, J. Hohlbein, Physical Biology, 16, 035001, 2019, [link]
Lactic acid bacteria (LAB) are frequently used in food fermentation and are invaluable for the taste and nutritional value of the fermentation end-product. To gain a better understanding of underlying biochemical and microbiological mechanisms and cell-to-cell variability in LABs, single-molecule techniques such as single-particle tracking photo-activation localization microscopy (sptPALM) hold great promises but are not yet employed due to the lack of detailed protocols and suitable assays.
Here, we qualitatively test various fluorescent proteins including variants that are photoactivatable and therefore suitable for sptPALM measurements in Lactococcus lactis, a key LAB for the dairy industry. In particular, we fused PAmCherry2 to dCas9 allowing the successful tracking of single dCas9 proteins, whilst the dCas9 chimeras bound to specific guide RNAs retained their gene silencing ability in vivo. The diffusional information of the dCas9 without any targets showed different mechanistic states of dCas9: freely diffusing, bound to DNA, or transiently interacting with DNA. The capability of performing sptPALM with dCas9 in L. lactis can lead to a better, general understanding of CRISPR-Cas systems as well as paving the way for CRISPR-Cas based interrogations of cellular functions in LABs.