CRISPR RNA or crRNA is a

RNA transcript Transcription is the process of copying a segment of DNA into RNA. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). Other segments of DNA are copied into RNA molecules calle ...

from the CRISPR locus.

CRISPR-Cas CRISPR () (an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragments of bact ...

(clustered, regularly interspaced short palindromic repeats - CRISPR associated systems) is an adaptive immune system found in

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

and archaea to protect against

mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...

, like

virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...

es,

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...

s, and

transposons A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Trans ...

. The CRISPR locus contains a series of repeats interspaced with unique spacers. These unique spacers can be acquired from MGEs. 13 Hegasy CRISPR pre crRNA Wiki E CCBYSA

13 Hegasy CRISPR pre crRNA Wiki E CCBYSA

Pre-crRNA is formed after the transcription of the CRISPR locus and before being processed by Cas proteins. Mature crRNA transcripts contain a partial conserved section of repeat and a sequence of spacer that is complementary to the target DNA. crRNA forms an effector complex with a single nuclease or multiple Cas proteins called a Cascade (CRISPR-associated complex for antiviral defense). Once the effector complex is formed a Cas

nuclease A nuclease (also archaically known as nucleodepolymerase or polynucleotidase) is an enzyme capable of cleaving the phosphodiester bonds between nucleotides of nucleic acids. Nucleases variously effect single and double stranded breaks in their t ...

or single effector protein will cause interference guided by the crRNA match.

Function

Type-I

Type-I CRISPR systems are characterized by

Cas3 Cas3 is an ATP-dependent single-strand DNA (ssDNA) translocase/helicase enzyme that degrades DNA as part of CRISPR based immunity. Cas3 is a "signature" protein of class 1 CRISPR systems and functions in a complex known as CASCADE, with other ca ...

, a nuclease-

helicase Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separat ...

protein, and the multi-subunit Cascade (CRISPR-associated complex for antiviral defense). The crRNA can form a complex with the Cas proteins in the Cascade and guide the complex to the target DNA sequence. Cas3 is recruited for the nuclease-helicase activity. Typically in the Cascade, Cas6 generates the mature crRNAs while Cas5 and Cas7 process and stabilize the crRNA.

Type-II

Type-II CRISPR systems are characterized by the single signature nuclease

Cas9 Cas9 (CRISPR associated protein 9, formerly called Cas5, Csn1, or Csx12) is a 160 kilodalton protein which plays a vital role in the immunological defense of certain bacteria against DNA viruses and plasmids, and is heavily utilized in genetic ...

. In type-II CRISPR systems crRNA and

tracrRNA In molecular biology, trans-activating crispr RNA (tracrRNA) is a small ''trans''-encoded RNA. It was first discovered by Emmanuelle Charpentier in her study of human pathogen ''Streptococcus pyogenes'', a type of bacteria that causes harm to human ...

(trans-activating CRISPR RNA) can form a complex known as the guide RNA or gRNA. The crRNA within the gRNA is what matches up with the target sequence or protospacer after the PAM is found. Once the match is made Cas9 will make a double-stranded break. The Stages of CRISPR immunity

Type-III

Type-III CRISPR systems are characterized by Cas10, an RNA cleaving protein. Similar to type-I, a large subunit effector complex is formed and crRNA guides the complex to the target sequence. Cas6 helps to generate the mature crRNA.

Type-IV

Type-IV CRISPR systems do not have an effector nuclease and are associated with

s and

prophage A prophage is a bacteriophage (often shortened to "phage") genome that is integrated into the circular bacterial chromosome or exists as an extrachromosomal plasmid within the bacterial cell. Integration of prophages into the bacterial host is the ...

s. A Cas6-like enzyme is associated with the maturation of the crRNA. Not all type-IV systems have a CRISPR locus and therefore do not have crRNA.

Type-V

Type-V CRISPR systems are characterized by Cas12, a nuclease that can cleave ssDNA, dsDNA, and

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

. Like Cas9, Cas12 is the single effector nuclease. Type-V systems process pre-crRNA without tracrRNA. The mature crRNA in complex with Cas12 target the DNA sequence of interest and cleave the DNA.

Type-VI

Type-VI CRISPR systems are characterized by Cas13, a single effector protein that targets RNA. Like the type-V system, Cas13 can process the pre-crRNA without tracrRNA. The mature crRNA in complex with Cas13 guides the complex to the target RNA and degrades it.

References

{{Reflist RNA