Type III CRISPR Systems

Introduction

Clustered Regularly Spaced Palendromic Repeats (CRISPR)are DNA sequences found throughout prokaryotes and archaea that, in conjunction with a variety of Cas enzymes, are responsible for anti-phage defense mechanisms (Barangou et al, 2007). Generally, foreign bacteriophage DNA is recognized by the enzymes Cas1 and Cas2 and incorporated into the CRISPR array within the host organisms’ genome. These portions of bacteriophage DNA are known as spacers. These spacers can then be transcribed into RNA and used as guides to direct the cleavage of future phage invaders. There exists a tremendous diversity of CRISPR systems, there are two distinct classes of CRISPR systems each with three types of CRISPR/Cas systems containing their own specialized Cas enzymes and modes of action. The type II CRISPR system, encoding a Cas9 endonuclease, is the best characterized of the CRISPR systems due to its prevalence as a gene editing tool.