How is Cas prevented from cutting its own CRISPR locus?

Posted June 22, 2020

Answer

The PAM is a component of the invading virus or plasmid, but is not found in the bacterial host genomes. Besides, the spacers that are derived from the invading virus and later integrated into the CRISPR locus also do not contain a PAM sequence. Therefore, CRISPR itself does not contain PAM sequences. Since Cas9 can cleave DNA targets only when there is a PAM sequence present, the CRISPR locus of host cells are prevented from cutting by its Cas9.

Additional resources

Helixyte™ Green *10,000X Aqueous PCR Solution*

6-ROX glycine *25 uM fluorescence reference solution for PCR reactions*

Doudna, J. A., & Charpentier, E. (2014). The new frontier of genome engineering with CRISPR-Cas9. Science, 346(6213), 1258096.

Ran, F. A., Hsu, P. D., Wright, J., Agarwala, V., Scott, D. A., & Zhang, F. (2013). Genome engineering using the CRISPR-Cas9 system. Nature protocols, 8(11), 2281.