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.
Fantastic news: Our miCube microscopy platform and the recent publication “Visualisation of dCas9 target search in vivo using an open-microscopy framework” [link] was featured in Nature Methods [link]. Thank you Dr. Strack!
, Nucleic Acid Research, gkz797, 2019, [
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.
Šarūnė recently started her Erasmus+ internship in the group. She will characterise a variety of fluidic devices that we got our hands on including devices for high throughput smFRET screening, trapping of bacteria and establishing DNA curtains. Elmar started his MSc thesis and will utilise new DNA constructs for smFRET based studies of ARF transcription factor binding.
Auxin controls numerous growth processes in land plants through a gene expression system that modulates ARF transcription 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. Expression patterns of both ARF proteins define developmental zones where auxin response is permitted, quantitatively tuned, or prevented. This fundamental design likely represents the ancestral system, and formed the basis for inflated, complex systems.
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.
L. lactis PAINTed with Nile-Red. Work by Ben Tumulero and others. ImageJ/FIJI plugin by Andrey Aristov (Pasteur) based on scripts by Eugene Katrukha (Utrecht).