ABSTRACT
The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) protein system is an adaptive immune system in prokaryotic organisms. It defends against invading bacteriophages and conjugative plasmids. 1 Besides the genes that encode Cas proteins, CRISPR loci harbor short repeats (usually 20–50 bp) separated by nonrepeating spacers (21–72 bp) derived from the invading DNA (Figure 16.1). New spacers are acquired over time and when expressed, guide the Cas effector protein(s) to cleave the invading DNA thus providing immunity against reinfection. Diverse CRISPR-Cas systems have been identified from many bacteria and are now divided into two classes (1 and 2) and further subdivided into six types (I–VI). 2 Class 1 includes type I, III, and IV CRISPR-Cas systems that use multisubunit effector complexes to degrade foreign nucleic acids. Class 2 includes type II, V, and VI and uses a single large Cas protein for the same purposes. Distinct signature Cas proteins are classified in different types, for example, Cas3 in type I, Cas9 in type II, and Cas10 in type III. Understanding the molecular mechanisms of these CRISPR-Cas systems launched the birth of a diversity of novel, programmable CRISPR-based tools for genome editing, 3 – 5 transcriptional control, 6 nucleic acid detection, 7 , 8 as well as DNA cloning and imaging. 9 , 10 Here we summarize the history of CRISPR-Cas biology including major discoveries that led to our mechanistic understanding of these systems and their subsequent translational applications (Figure 16.2).
