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Anti-CRISPR type II subtype A6 AcrIIA6 is an anti-CRISPR (Acr) protein which was discovered via a self-targeting method that inhibits the type II-A CRISPR-Cas system (Csy). It is the largest known type II-A Acr, and it contains a helix-turn-helix (HTH) motif that is part of its dimerization interface. AcrIIA6 inhibits target DNA binding to Cas9 via an allosteric mechanism and can induce Cas9 dimerization. The protospacer adjacent motif (PAM) and AcrIIA6 recognition sites are structurally close and allosterically linked. AcrIIA6 affects Cas9 conformational dynamics associated with PAM binding. Almost all type II Acrs characterized to date directly interact with the Cas9 endonuclease, although by distinct mechanisms. The type II CRISPR/Cas subtype has a distinct crRNA-guided surveillance complex encoded by cas9 (formerly csn1), cas1, cas2, and csn2 (for type IIA) or cas4 (for type IIB) genes, all located in a single transcriptional unit directly upstream of the CRISPR locus. Cleavage of the DNA target in type II systems is carried out by Cas9 which is an RNA-guided double-stranded DNase with two independent nuclease domains, HNH and RuvC. Due to the reliance of the type II system on a single protein for function, Cas9 homologs derived from different subtypes and species have been utilized for numerous gene editing applications. CRISPR-Cas immune systems are used by certain prokaryotes and archaea to resist the invasion of foreign nucleic acids such as phages or plasmids. Anti-CRISPRs are small proteins which are the natural inhibitors for CRISPR-Cas systems; encoded on bacterial and archaeal viruses, they allow the virus to evade host CRISPR-Cas systems. The CRISPR-Cas-mediated adaptive immune response can be divided into three steps, including the acquisition of spacer derived from invading nucleic acids, crRNA processing, and target degradation. Theoretically, Acr proteins could suppress any step to disrupt the CRISPR-Cas system. Acr proteins are diverse with no common sequence or structural motif, and they inhibit a wide range of CRISPR-Cas systems with various inhibition mechanisms. CRISPR-Cas systems are divided into two classes (1 and 2) and six types (class 1: types I, III and IV; class 2: types II, V and VI). Acr families are named for their type and subtype which are numbered sequentially as they are discovered. |
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