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Science: Revealing A Versatile V-type CRISPR-Cas System


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Science: Revealing A Versatile V-type CRISPR-Cas System Inquiry

The CRISPR/Cas system of archaea and bacteria protects their host from phage and other mobile genetic elements. According to the latest classification standards, the CRISPR/Cas system can be divided into two categories: the first type of CRISPR/Cas system and the second type of CRISPR/Cas system.

 

The first type of CRISPR/Cas system is divided into a type I CRISPR/Cas system (tag gene is Cas3), a type III CRISPR/Cas system (tag gene is Cas10), and a type IV CRISPR/Cas system (tag gene is Csf1). Type I CRISPR/Cas system can be further divided into IA (tag genes are Cas8a, Cas5), IB (tag gene is Cas8b), IC (tag gene is Cas8c), ID (tag gene is Cas10d), IE (tag gene are Cse1, Cse2), IF (tag genes are Csy1, Csy2, and Csy3), IU (tag gene is GSU0054); type III CRISPR/Cas system can be further divided into III-A (tag gene is Csm2), III-B (tag gene is Cmr5), III-C (tag gene is Cas10 or Csx11), III-D (tag gene is Csx10); the type IV CRISPR/Cas system can be further divided into IV-A and IV-B.

 

 

The second type of CRISPR/Cas system is divided into type II CRISPR/Cas system, whose tag gene is Cas9; V-type CRISPR/Cas system, whose tag genes are Cas12a (formerly known as Cpf1), Cas12b (formerly known as C2c1), and Cas12c (formerly known as C2c3); type VI CRISPR/Cas system, whose tag genes are Cas13a (formerly known as C2c2), Cas13b and Cas13c. Type II CRISPR/Cas system can be further divided into II-A (tag gene is Csn2), IIB (tag gene is Cas4) and IIC (Csn2 and Cas4 absent), V-type CRISPR/Cas system can be further divided into VA (tag gene is Cas12a), VB (tag gene is Cas12b), VC (tag gene is Cas12c), and VU (tag gene is TnpB-like). The Type VI CRISPR/Cas system can be further divided into VI-A (tag gene is Cas13a) and VI-B (tag gene is Cas13b).

 

When expressed, the type I CRISPR/Cas system produces three parts: Cas3, Cascade complex (composed of Cse1, Cse2, Cas7, Cas5, and Case6e) and pre-crRNA, then pre-crRNA is sheared by Case6e of the Cascade complex to obtain mature crRNA, and the mature crRNA will continue to bind to the Cascade complex to function. The most commonly used CRISPR/Cas9 and CRISPR/Cas12a systems belong to the Type II CRISPR/Cas system and the V-A Type CRISPR/Cas system, respectively. Cas12a (formerly known as Cpf1) is a CRISPR-related nuclease and has a close relationship with the Cas9 nuclease. Today, Cas12a and Cas9 are widely used in synthetic genomics engineering, agricultural genomics and biomedical fields.

 

The V-type CRISPR-Cas system can be distinguished by a single RNA-directed effector protein Cas12. Cas12 contains a RuvC domain at the carboxy terminus. Although the effector protein Cas12a of the VA-type CRISPR-Cas and the effector protein Cas12b of the VB-type CRISPR-Cas have been studied in detail, the different domain structures of the Cas12 protein not studied and the different RuvC sequences indicate that the presence of unexplored V type CRISPR-Cas is functionally diverse.

 

In a new study, researchers from Arbor Biotechnologies in the United States identified other members of the Cas12 protein: Cas12c, Cas12g, Cas12h, and Cas12i exhibited RNA-directed double-stranded DNA (dsDNA) interference activity. Cas12i exhibits significantly different cleavage efficiencies for the complementary and non-complementary strands of the crRNA spacer sequence, primarily leading to dsDNA nicking. As a ribonuclease (RNase), Cas12g exhibits RNA-directed targeted cleavage RNA activity, in addition with activity with non-specifically trans-cleaving single-stranded DNA (ssDNA).

 

This study reveals a multi-energy diversity in the different evolutionary pathways of the V-type CRISPR-Cas, which extends the CRISPR toolbox.

 

 

Reference

Winston X. Yan et al. Functionally diverse type V CRISPR-Cas systems, 2018, doi:10.1126/science.aav7271.

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