How Does the CRISPR System Work, continued

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The page below is a sample from the LabCE course CRISPR: From Nature to Bench and Bedside. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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How Does the CRISPR System Work, continued

tracrRNA
Some types of bacteria harbor an additional RNA tool called “tracrRNA” to assist in processing crRNA.5 Single guide RNA (sgRNA) has two parts: the tracrRNA is the binding scaffold for Cas9 endonuclease, whereas the crRNA is a 17–20 nucleotide sequence that is complementary to the phage DNA (re: the target).
Regarding RNA sequence, tracrRNA is complementary to the unprocessed crRNA. With the help of an enzyme, tracrRNA forms a complex with crRNA, which we can designate as “tracrRNA::crRNA”. This RNA complex is of paramount significance for revolutionizing CRISPR from nature to lab-modified CRISPR pioneered by two scientists who were awarded the Nobel Prize in 2020. A detailed discussion of their research is presented in the next section.
The figure on the right illustrates the transcription from phage DNA to crRNA and how tracrRNA is complementary to unprocessed crRNA.
5. Doudna, J.A. (2022). CRISPR in Nature. In M.L. Hochstrasser et al. (Eds.) CRISPRpedia. Berkeley: Innovative Genomics Institute, University of California, Berkeley. https://innovativegenomics.org/crisprpedia/crispr-in-nature/

Figure 1. Transcription from phage DNA to crRNA and how tracrRNA is complementary to unprocessed crRNA