Molecule of the Month: Cascade and CRISPR
Cascade and CRISPR help bacteria remember how to fight viral infection
Archive of Infection
Cas9 and Cures
Exploring the Structure
Cascade (PDB entry 4qyz)
PDB entry 4qyz captures Cascade in action. The structure includes the strand of CRISPR RNA (red) and a short piece of the viral DNA (yellow) after it has been unwound and recognized. The structure revealed a surprising but very logical structure for the RNA and DNA. The RNA is stretched open in a long spiral groove in Cascade, and the DNA binds side-by-side, instead of in the classical double helix. To explore this amazing structure in more detail, click on the image for an interactive JSmol.
Topics for Further Discussion
- When reading articles about CRISPR sequences, watch out for some of the terminology, because it can be confusing. For instance, the term "spacer" is often used to refer to the short pieces of viral DNA that are stored in the CRISPR, and "repeat" is used to refer to the short repeated sequences separating each piece of viral DNA.
- Many of these large CRISPR/Cas complexes have been characterized by electron microscopy. For instance, to see the structure of a type III complex (which is different from type I Cascade and type II Cas9) take a look at the EMDataBank.
January 2015, David Goodselldoi:10.2210/rcsb_pdb/mom_2015_1