SMART accession number:SM01101
Description: This domain forms an anti-parallel beta strand structure with flanking alpha helical regions.
Interpro abstract (IPR010179):

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are a family of DNA direct repeats separated by regularly sized non-repetitive spacer sequences that are found in most bacterial and archaeal genomes [(PUBMED:17442114)]. CRISPRs appear to provide acquired resistance against mobile genetic elements (viruses, transposable elements and conjugative plasmids. CRISPR clusters contain sequences complementary to antecedent mobile elements and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). Cas3 plus Cascade participate in CRISPR interference, the third stage of CRISPR immunity. It anneals and unwinds R-loops (in which crRNA binds the target DNA, displacing the noncomplementary strand). During this process, the unwinding requires ATP, while the annealing does not [(PUBMED:17379808), (PUBMED:16545108), (PUBMED:21699496)].

Differences in the number and type of spacers between CRISPR repeats correlate with phage sensitivity. It is thought that following phage infection, bacteria integrate new spacers derived from phage genomic sequences, and that the removal or addition of particular spacers modifies the phage-resistance phenotype of the cell. Therefore, the specificity of CRISPRs may be determined by spacer-phage sequence similarity.

In addition, there are many protein families known as CRISPR-associated sequences (Cas), which are encoded in the vicinity of CRISPR loci [(PUBMED:16292354)]. CRISPR/cas gene regions can be quite large, with up to 20 different, tandem-arranged cas genes next to a CRISPR cluster or filling the region between two repeat clusters. Cas genes and CRISPRs are found on mobile genetic elements such as plasmids, and have undergone extensive horizontal transfer. Cas proteins are thought to be involved in the propagation and functioning of CRISPRs. Some Cas proteins show similarity to helicases and repair proteins, although the functions of most are unknown. Cas families can be divided into subtypes according to operon organisation and phylogeny.

This entry represents the Cse3 (CRISPR/Cas Subtype Ecoli protein 3) family of Cas proteins. The Thermus thermophilus HB8 family member has been crystallised and found to have a structure consisting of two domains with opposing parallel beta-sheets, known as a beta-sheet platform [(PUBMED:16672237)]. This structure is similar to those found in the sex-lethal protein and poly(A)-binding protein and is consistent with an RNA-binding function.

Family alignment:
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There are 699 CRISPR_assoc domains in 699 proteins in SMART's nrdb database.

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