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‘Disintegration’—the reversal of transposon DNA integration at a target site—is regarded as an abortive off-pathway reaction. Here, we challenge this view with a biochemical investigation of the mechanism of protospacer insertion, which is mechanistically analogous to DNA transposition, by the Streptococcus pyogenes Cas1-Cas2 complex. In supercoiled target sites, the predominant outcome is the disintegration of one-ended insertions that fail to complete the second integration event. In linear target sites, one-ended insertions far outnumber complete protospacer insertions. The second insertion event is most often accompanied by the disintegration of the first, mediated either by the 3′-hydroxyl exposed during integration or by water. One-ended integration intermediates may mature into complete spacer insertions via DNA repair pathways that are also involved in transposon mobility. We propose that disintegration-promoted integration is functionally important in the adaptive phase of CRISPR-mediated bacterial immunity, and perhaps in other analogous transposition reactions. 

Abstract ¹⁹F magnetic resonance (MR) based detection coupled with well‐designed inorganic systems shows promise in biological investigations. Two proof‐of‐concept inorganic probes that exploit a novel mechanism for¹⁹F MR sensing based on converting from low‐spin (S=0) to high‐spin (S=1) Ni²⁺are reported. Activation of diamagneticNiL₁andNiL₂by light or β‐galactosidase, respectively, converts them into paramagneticNiL₀, which displays a single¹⁹F NMR peak shifted by >35 ppm with accelerated relaxation rates. This spin‐state switch is effective for sensing light or enzyme expression in live cells using¹⁹F MR spectroscopy and imaging that differentiate signals based on chemical shift and relaxation times. This general inorganic scaffold has potential for developing agents that can sense analytes ranging from ions to enzymes, opening up diverse possibilities for¹⁹F MR based biosensing.