Adaptation of clustered regularly interspaced short palindromic repeats (CRISPR) arrays is a crucial process responsible for the unique, adaptive nature of CRISPR-Cas immune systems. The acquisition of new CRISPR spacers from mobile genetic elements has previously been studied for several types of CRISPR-Cas systems. In this study, we used a high-throughput sequencing approach to characterize CRISPR adaptation of the type V-A system from Francisella novicida and the type V-B system from Alicyclobacillus acidoterrestris. In contrast to other class 2 CRISPR-Cas systems, we found that for the type V-A and V-B systems, the Cas12 nucleases are dispensable for spacer acquisition, with only Cas1 and Cas2 (type V-A) or Cas4/1 and Cas2 (type V-B) being necessary and sufficient. Whereas the catalytic activity of Cas4 is not essential for adaptation, Cas4 activity is required for correct protospacer adjacent motif selection in both systems and for prespacer trimming in type V-A. In addition, we provide evidence for acquisition of RecBCD-produced DNA fragments by both systems, but with spacers derived from foreign DNA being incorporated preferentially over those derived from the host chromosome. Our work shows that several spacer acquisition mechanisms are conserved between diverse CRISPR-Cas systems, but also highlights unexpected nuances between similar systems that generally contribute to a bias of gaining immunity against invading genetic elements.

Prokaryotes adapt to challenges from mobile genetic elements by integrating spacers derived from foreign DNA in the CRISPR array¹. Spacer insertion is carried out by the Cas1–Cas2 integrase complex^2–4. A substantial fraction of CRISPR–Cas systems use a Fe–S cluster containing Cas4 nuclease to ensure that spacers are acquired from DNA flanked by a protospacer adjacent motif (PAM)^5,6 and inserted into the CRISPR array unidirectionally, so that the transcribed CRISPR RNA can guide target searching in a PAM-dependent manner. Here we provide a high-resolution mechanistic explanation for the Cas4-assisted PAM selection, spacer biogenesis and directional integration by type I-G CRISPR in Geobacter sulfurreducens, in which Cas4 is naturally fused with Cas1, forming Cas4/Cas1. During biogenesis, only DNA duplexes possessing a PAM-embedded 3′-overhang trigger Cas4/Cas1–Cas2 assembly. During this process, the PAM overhang is specifically recognized and sequestered, but is not cleaved by Cas4. This ‘molecular constipation’ prevents the PAM-side prespacer from participating in integration. Lacking such sequestration, the non-PAM overhang is trimmed by host nucleases and integrated to the leader-side CRISPR repeat. Half-integration subsequently triggers PAM cleavage and Cas4 dissociation, allowing spacer-side integration. Overall, the intricate molecular interaction between Cas4 and Cas1–Cas2 selects PAM-containing prespacers for integration and couples the timing of PAM processing with the stepwise integration to establish directionality.

Vink et al. tracked single CRISPR RNA-surveillance complexes (Cascade) in the native host cell and determined the influence of Cascade copy numbers, PAM scanning speed, and the presence of CRISPR arrays and transcription on their ability to find and clear invading mobile genetic elements from the cell.

In two recent studies in Molecular Cell, Wright et al. (2019) report complete spacer integration by a Cas1 mini-integrase and Edraki et al. (2019) describe accurate genome editing by a small Cas9 ortholog with less stringent PAM requirements.

Integrating short DNA fragments at the correct leader-repeat junction is key to successful CRISPR-Cas memory formation. The Cas1-2 proteins are responsible to carry out this process. However, the CRISPR adaptation process additionally requires a DNA element adjacent to the CRISPR array, called leader, to facilitate efficient localization of the correct integration site. In this work, we introduced the core CRISPR adaptation genes cas1 and cas2 from the Type I-D CRISPR-Cas system of Synechocystis sp. 6803 into Escherichia coli and assessed spacer integration efficiency. Truncation of the leader resulted in a significant reduction of spacer acquisition levels and revealed the importance of different conserved regions for CRISPR adaptation rates. We found three conserved sequence motifs in the leader of I-D CRISPR arrays that each affected spacer acquisition rates, including an integrase anchoring site. Our findings support the model in which the leader sequence is an integral part of type I-D adaptation in Synechocystis sp. acting as a localization signal for the adaptation complex to drive CRISPR adaptation at the first repeat of the CRISPR array.

CRISPR–Cas systems are able to acquire immunological memories (spacers) from bacteriophages and plasmids in order to survive infection; however, this often occurs at low frequency within a population, which can make it difficult to detect. Here we describe CAPTURE (CRISPR adaptation PCR technique using reamplification and electrophoresis), a versatile and adaptable protocol to detect spacer-acquisition events by electrophoresis imaging with high-enough sensitivity to identify spacer acquisition in 1 in 10 ⁵ cells. Our method harnesses two simple PCR steps, separated by automated electrophoresis and extraction of size-selected DNA amplicons, thus allowing the removal of unexpanded arrays from the sample pool and enabling 1,000-times more sensitive detection of new spacers than alternative PCR protocols. CAPTURE is a straightforward method that requires only 1 d to enable the detection of spacer acquisition in all native CRISPR systems and facilitate studies aimed both at unraveling the mechanism of spacer integration and more sensitive tracing of integration events in natural ecosystems.

Microbes have the unique ability to acquire immunological memories from mobile genetic invaders to protect themselves from predation. To confer CRISPR resistance, new spacers need to be compatible with a targeting requirement in the invader's DNA called the protospacer adjacent motif (PAM). Many CRISPR systems encode Cas4 proteins to ensure new spacers are integrated that meet this targeting prerequisite. Here we report that a gene fusion between cas4 and cas1 from the Geobacter sulfurreducens I-U CRISPR-Cas system is capable of introducing functional spacers carrying interference proficient TTN PAM sequences at much higher frequencies than unfused Cas4 adaptation modules. Mutations of Cas4-domain catalytic residues resulted in dramatically decreased naïve and primed spacer acquisition, and a loss of PAM selectivity showing that the Cas4 domain controls Cas1 activity. We propose the fusion gene evolved to drive the acquisition of only PAM-compatible spacers to optimize CRISPR interference.

CRISPR-Cas systems adapt their immunological memory against their invaders by integrating short DNA fragments into clustered regularly interspaced short palindromic repeat (CRISPR) loci. While Cas1 and Cas2 make up the core machinery of the CRISPR integration process, various class I and II CRISPR-Cas systems encode Cas4 proteins for which the role is unknown. Here, we introduced the CRISPR adaptation genes cas1, cas2, and cas4 from the type I-D CRISPR-Cas system of Synechocystis sp. 6803 into Escherichia coli and observed that cas4 is strictly required for the selection of targets with protospacer adjacent motifs (PAMs) conferring I-D CRISPR interference in the native host Synechocystis. We propose a model in which Cas4 assists the CRISPR adaptation complex Cas1-2 by providing DNA substrates tailored for the correct PAM. Introducing functional spacers that target DNA sequences with the correct PAM is key to successful CRISPR interference, providing a better chance of surviving infection by mobile genetic elements. Kieper et al. demonstrate that the ubiquitous protein Cas4 assists Cas1 and Cas2 in the selection of new CRISPR spacers with a PAM licensing efficient CRISPR interference.