Visualisation of dCas9 target search in vivo using an open-microscopy framework
Martens, Koen J.A. ; Beljouw, Sam P.B. van; Els, Simon van der; Vink, Jochem N.A. ; Baas, Sander ; Vogelaar, George A. ; Brouns, Stan J.J. ; Baarlen, Peter van; Kleerebezem, Michiel ; Hohlbein, Johannes - \ 2019
Nature Communications 10 (2019)1. - ISSN 2041-1723
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.
Phasor based single-molecule localization microscopy in 3D (pSMLM-3D): An algorithm for MHz localization rates using standard CPUs
Martens, Koen J.A. ; Bader, Arjen N. ; Baas, Sander ; Rieger, Bernd ; Hohlbein, Johannes - \ 2018
Journal of Chemical Physics 148 (2018)12. - ISSN 0021-9606
We present a fast and model-free 2D and 3D single-molecule localization algorithm that allows more than 3 × 10 6 localizations per second to be calculated on a standard multi-core central processing unit with localization accuracies in line with the most accurate algorithms currently available. Our algorithm converts the region of interest around a point spread function to two phase vectors (phasors) by calculating the first Fourier coefficients in both the x- and y-direction. The angles of these phasors are used to l ocalize the center of the single fluorescent emitter, and the ratio of the magnitudes of the two phasors is a measure for astigmatism, which can be used to obtain depth information (z-direction). Our approach can be used both as a stand-alone algorithm for maximizing localization speed and as a first estimator for more time consuming iterative algorithms.