资源环境科技发展态势分析平台

CRISPR-Cas immunity protects prokaryotes against invading genetic elements(1). It uses the highly conserved Cas1-Cas2 complex to establish inheritable memory (spacers)(2-5). How Cas1-Cas2 acquires spacers from foreign DNA fragments (prespacers) and integrates them into the CRISPR locus in the correct orientation is unclear(6,7). Here, using the high spatiotemporal resolution of single-molecule fluorescence, we show that Cas1-Cas2 selects precursors of prespacers from DNA in various forms-including single-stranded DNA and partial duplexes-in a manner that depends on both the length of the DNA strand and the presence of a protospacer adjacent motif (PAM) sequence. We also identify DnaQ exonucleases as enzymes that process the Cas1-Cas2-loaded prespacer precursors into mature prespacers of a suitable size for integration. Cas1-Cas2 protects the PAM sequence from maturation, which results in the production of asymmetrically trimmed prespacers and the subsequent integration of spacers in the correct orientation. Our results demonstrate the kinetic coordination of prespacer precursor selection and PAM trimming, providing insight into the mechanisms that underlie the integration of functional spacers in the CRISPR loci.

Cas1-Cas2 selects precursor prespacers from DNA fragments in a length- and PAM-sequence-dependent manner, and these precursors are trimmed by DnaQ exonucleases to enable integration into the CRISPR locus in the correct orientation.

Flash Joule heating of inexpensive carbon sources is used to produce gram-scale quantities of high-quality graphene in under a second, without the need for a furnace, solvents or reactive gases.

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials(1,2) is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations(3-5). Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co3O4 nanocube core that carries a Mn3O4 shell on each facet. The individual shells are symmetry-related interconnected grains(6), and the large geometric misfit between adjacent tetragonal Mn3O4 grains results in tilt boundaries at the sharp edges of the Co3O4 nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase(7). Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation(8-10). With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.