Bacterial adaptive immune systems use CRISPRs (clustered regularly interspaced short palindromic repeats) and CRISPR-associated (Cas) proteins for RNA-guided nucleic acid cleavage1,2. Although most prokaryotic adaptive immune systems generally target DNA substrates3,4,5, type III and VI CRISPR systems direct interference complexes against single-stranded RNA substrates6,7,8,9. In type VI systems, the single-subunit C2c2 protein functions as an RNA-guided RNA endonuclease (RNase)9,10. How this enzyme acquires mature CRISPR RNAs (crRNAs) that are essential for immune surveillance and how it carries out crRNA-mediated RNA cleavage remain unclear. Here we show that bacterial C2c2 possesses a unique RNase activity responsible for CRISPR RNA maturation that is distinct from its RNA-activated single-stranded RNA degradation activity. These dual RNase functions are chemically and mechanistically different from each other and from the crRNA-processing behaviour of the evolutionarily unrelated CRISPR enzyme Cpf1 (ref. 11). The two RNase activities of C2c2 enable multiplexed processing and loading of guide RNAs that in turn allow sensitive detection of cellular transcripts.
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We thank the QB3 MacroLab for assistance with cloning of C2c2 constructs; N. Ma and K. Zhou for technical assistance; S. N. Floor, S. C. Strutt, A. V. Wright and M. L. Hochstrasser for critical reading of the manuscript; and members of the Doudna, Cate and Tjian laboratories for discussions. S.C.K. acknowledges support from the National Science Foundation Graduate Research Fellowship Program; M.R.O. is a recipient of a C. J. Martin Overseas Early Career Fellowship from the National Health and Medical Research Council of Australia. This work was supported in part by a Frontiers Science award from the Paul Allen Institute to J.A.D., the National Science Foundation (MCB-1244557 to J.A.D.), the California Institute for Regenerative Medicine (CIRM, RB4-06016 to R.T.), and the National Institutes of Health (P50-GM102706 to J.H.D.C). R.T. and J.A.D. are Investigators of the Howard Hughes Medical Institute. J.A.D. is a co-founder of Caribou Biosciences, Editas Medicine and Intellia Therapeutics and a scientific advisor to Caribou, Intellia, eFFECTOR Therapeutics, and Driver. A.E.S., M.R.O., S.C.K., J.H.D.C. and J.A.D. have filed a patent application related to this work.
Authors A.E.S., M.R.O., S.C.K., J.H.D.C. and J.A.D. are inventors on a related patent application.
Nature thanks M. White, J. Wilusz and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, Maximum-likelihood phylogenetic reconstuction of C2c2 proteins. Leaves include GI protein numbers and organism of origin; bootstrap support values, out of 100 resamplings, are presented for inner split. Scale is in substitutions per site. b, Multiple sequence alignment of the three analysed homologues of C2c2; coordinates are based on LbuC2c2.
All C2c2 homologues were expressed in E. coli as His-MBP fusions and purified by a combination of affinity, ion exchange and size exclusion chromatography. The Ni+ affinity tag was removed by incubation with TEV protease. a, b, Representative SDS–PAGE gels of chromatography fractions are shown. c, The chromatogram from Superdex 200 (16/60) column demonstrating that C2c2 elutes as a single peak, devoid of nucleic acid. d, SDS–PAGE analysis of purified proteins used.
a, Cleavage site mapping of LseC2c2 and LshCc2c2 cleavage of a single cognate pre-crRNA array. Cleavage reactions were performed with 100 nM C2c2 and <1 nM pre-crRNA. b–i, Re-analysis of LshC2c2 (b–f) and LseC2c2 (g–i) CRISPR array RNA sequencing experiments from ref. 10 (supplementary figs S7 and S5 of ref. 10, respectively). All reads (b, g) and filtered reads (55 nucleotides or less; as per original analysis10; c, h) were stringently aligned to each CRISPR array using Bowtie2 (see Methods). Detailed views of individual CRISPR repeat-spacers are shown for Lsh (d–f) and Lse (i). Differences in 5′ end pre-crRNA processing are indicated by arrows below each sequence. BAM alignment files of our analysis are available in Supplementary Information. This mapping clearly indicates that the 5′ ends of small RNA sequencing reads generated from Lsh pre-crRNAs map to a position 2 nucleotides from the base of the predicted hairpin, in agreement with our in vitro processing data (a). This pattern holds for all mature crRNAs detected from both native expression in L. shahii and heterologous expression in E. coli (data not shown, BAM file available in Supplementary Information). Unfortunately, the LseC2c2 crRNA sequencing data (used in g–i) is less informative owing to low read depth, and each aligned crRNA exhibits a slightly different 5′ end with little obvious uniformity. The mapping for one of the processed repeats (repeat-spacer 2; i) is in agreement with our data but only with low confidence due to the insufficient read depth.
Extended Data Figure 4 Pre-crRNA processing by C2c2 is spacer-sequence independent, can occur on tandem crRNA arrays, is affected by mutations in the 5′ flanking region of the pre-cRNA and produces a 3′ phosphate product.
a, Cleavage site mapping of LbuC2c2 cleavage of a tandem pre-crRNA array. Cleavage reactions were performed with 100 nM LbuC2c2 and <1 nM pre-crRNA. A schematic of cleavage products is depicted on right, with arrows indicating the mapped C2c2 cleavage products. b, LbuC2c2 4-mer mutant pre-crRNA processing data demonstrating the importance of the 5′ single-stranded flanking region for efficient pre-crRNA processing. Percentage of pre-crRNA processing was measured after 1 h (mean ± s.d., n = 3). c, Representative LbuC2c2 pre-crRNA cleavage time course demonstrating that similar rates of pre-crRNA processing occur independent of crRNA spacer sequence pseudo-first-order rate constants (kobs) (mean ± s.d.) are 0.07 ± 0.04 min−1 and 0.08 ± 0.04 min−1 for spacer A and spacer λ2, respectively. d, End group analysis of cleaved RNA by T4 polynucleotide kinase (PNK) treatment. Standard processing assay conditions were used to generate cleavage product, which was then incubated with PNK for 1 h to remove any 2′,3′-cyclic phosphates/3′ monophosphates. Retarded migration of band indicates removal of the charged, monophosphate from the 3′ end of radiolabelled 5′ product.
Extended Data Figure 5 LbuC2c2 catalyses guide-dependent ssRNA degradation on cis and trans targets.
a, Schematic of the two modes of C2c2, guide-dependent ssRNA degradation. b, Cleavage of two distinct radiolabelled ssRNA substrates, A and B, by LbuC2c2. Complexes of 100 nM C2c2 and 50 nM crRNA were pre-formed at 37 °C, and reaction was initiated upon addition of <1 nM 5′-labelled target RNA at 25 °C. Trans cleavage reactions contained equimolar (<1 nM) concentrations of radiolabelled non-guide-complementary substrate, and unlabelled on-target ssRNA. For multiple ssRNA substrates, we observed that LbuC2c2 catalysed efficient cleavage only when bound to the complementary crRNA, indicating that LbuC2c2–crRNA cleaves ssRNA in an RNA-guided fashion. This activity is hereafter referred to as on-target or cis-target cleavage. LbuC2c2-mediated cis cleavage resulted in a laddering of multiple products, with cleavage preferentially occurring before uracil residues, analogous to LshC2c2 (ref. 9). We repeated non-target cleavage reactions in the presence of unlabelled, on-target (crRNA-complementary) ssRNA. In contrast to non-target cleavage experiments performed in cis, we observed rapid degradation of non-target RNA in trans. The similar RNA cleavage rates and near-identical cleavage products observed for both cis on-target cleavage and trans non-target cleavage implicate the same nuclease centre in both activities. c, LbuC2c2 loaded with crRNA targeting spacer A was tested for cleavage activity under both cis (target A labelled) and trans (target B labelled in the presence of unlabelled target A) cleavage conditions in the presence of 25 mM EDTA.
a, ssRNA target cleavage assay conducted per Methods demonstrating LbuC2c2-mediated ‘cis’ cleavage of several radiolabelled ssRNA substrates with identical spacer-complementary sequences but distinct 5′ flanking sequences of variable length and nucleotide composition. Sequences of ssRNA substrates are shown to the right with spacer-complementary sequences for crRNA-A highlighted in yellow. Arrows indicate detected cleavage sites. Gel was cropped for clarity. It should be noted that the pattern of cleavage products produced on different substrates (for example, A.1 versus A.2 versus A.3) indicates that the cleavage site choice is primarily driven by a uracil preference and exhibits an apparent lack of exclusive cleavage mechanism within the crRNA-complementary target sequence, which is in contrast to what is observed for other class II CRISPR single effector complexes such as Cas9 and Cpf1 (refs 11, 21). Notably, the cleavage pattern observed for substrate A.0 hints at a secondary preference for polyG sequences. b, LbuC2c2 ssRNA target cleavage assay as per Methods, using a range of crRNAs that tile the length of the ssRNA target. The sequence of the ssRNA substrates used in this experiment is shown below the gel with spacer-complementary sequences for each crRNA highlighted in yellow. Arrows indicate predicted cleavage sites. Above each set of lanes, a small diagram indicates the location of the spacer sequence along the target (yellow box) and the cleavage products observed (red arrows) or absent (black arrows). Likewise, it should be noted that for every crRNA the cleavage product length distribution is very similar, again indicating an apparent lack of exclusive cleavage within the crRNA-bound sequence. The absence of a several cleavage products in a subset of the reactions might be explained by the presence of bound C2c2–crRNA on the ssRNA target, which could sterically occlude access to uracils by any cis (intramolecular) or trans (intermolecular) LbuC2c2 active sites. While proper analysis for protospacer flanking site (PFS) preference for LbuC2c2 is beyond the scope of this study, minimal impact of the 3′ flanking nucleotide was observed. Expected PFS base is noted in diagram next to each guide tested in red.
Extended Data Figure 7 Dependence of RNA targeting on crRNA variants, temperature and point mutations.
a, LbuC2c2 ssRNA target cleavage assay carried out, as per Methods with crRNAs possessing 16-, 20- or 24-nucleotide spacers. b, LbuC2c2 ssRNA target cleavage time-course carried out at either 25 °C or 37 °C as per the Methods. c, LbuC2c2 ssRNA target cleavage time course carried out as stated in the Methods with crRNAs possessing different 5′-flanking nucleotide mutations. Mutations are highlighted in red. One- to two-nucleotide 5′ extensions negligibly impacted cleavage efficiencies. By contrast, shortening the flanking region to 3 nucleotides slowed cleavage rates. d, Effect of point mutations on RNase activity of C2c2 in conserved residue mutants within HEPN motifs for ssRNA targeting.
a, Filter binding assays were conducted as described in the Methods to determine the binding affinity of mature crRNA-A_GG to LbuC2c2-WT, LbuC2c2-dHEPN1, LbuC2c2-dHEPN2, or LbuC2c2-dHEPN1/dHEPN2. The quantified data were fit to standard binding isotherms. Error bars represent the s.d. from three independent experiments. Measured dissociation constants from three independent experiments (mean ± s.d.) were 27.1 ± 7.5 nM (LbuC2c2-WT), 15.2 ± 3.2 nM (LbuC2c2-dHEPN1), 11.5 ± 2.5 nM (LbuC2c2-dHEPN2), and 43.3 ± 11.5 nM (LbuC2c2-dHEPN1/dHEPN2). b, Representative electrophoretic mobility shift assay for binding reactions between LbuC2c2-dHEPN1/dHEPN2: crRNA-A_GG and either ‘on-target’ A ssRNA or ‘off-target’ B ssRNA, as indicated. Three independent experiments were conducted as described in the Methods. The gel was cropped for clarity. c, Quantified binding data from b were fitted to standard binding isoforms. Error bars represent the s.d. from three independent experiments. Measured dissociation constants from three independent experiments (mean ± s.d.) were 1.62 ± 0.43 nM for ssRNA A and not determined (N.D.;>>10 nM) for ssRNA B. d, Filter binding assays were conducted as described in the Methods to determine the binding affinity of mature crRNA-A_GA to LbuC2c2-WT and LbuC2c2-R1079A. The quantified data were fit to standard binding isotherms. Error bars represent the s.d. from three independent experiments. Measured dissociation constants from three independent experiments (mean ± s.d.) were 4.65 ± 0.6 nM (LbuC2c2-WT) and 2.52 ± 0.5 nM (LbuC2c2-R1079A). It is of note that these binding affinities differ from a. This difference is accounted for in a slight difference in the 5′ sequence of the guide with panel a guides beginning with a 5′-G(G)CCA… and panel d 5`-G(A)CCA. While the native sequence guide (5′-G(A)CCA) binds tighter to LbuC2c2, no difference is seen in the RNA targeting efficiencies of these guide variants (Extended Data Fig. 6c).
a, LbuC2c2:crRNA-λ2 was incubated with RNAase-Alert substrate (Thermo-Fisher) and 100 ng HeLa total RNA in the presence of increasing amounts of λ2 ssRNA (0-1 nM) for 2 h at 37 °C. Fluorescence measurements were taken every 5 min. The 1 nM λ2 ssRNA reaction reached saturation before the first time point could be measured. Error bars represent the s.d. from three independent experiments. b, LbuC2c2–crRNA-λ4 or apo LbuC2c2 was incubated in HeLa total RNA for 2 h in the presence or absence of on-target activating λ4 ssRNA. Degradation of background small RNA was resolved on a small RNA chip in a Bioanalyzer 2100 as described in the Methods. Small differences are seen in the fragment profile of between apo LbuC2c2 and LbuC2c2:crRNA-λ4. By contrast, upon addition of the on-target ssRNA to the reaction, a notable broadening and shifting of the tRNA peak reveals extensive degradation of other structured and nonstructured RNA’s present in the reaction upon activation of LbuC2c2 trans activity.
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East-Seletsky, A., O’Connell, M., Knight, S. et al. Two distinct RNase activities of CRISPR-C2c2 enable guide-RNA processing and RNA detection. Nature 538, 270–273 (2016). https://doi.org/10.1038/nature19802
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