Distribution and mechanism of Type I-E CRISPR-Cas systems

Although the CRISPR type I system encompasses six different subtypes (I-A to I-F), only three subtypes have been studied in detail to date. This review includes an analysis of the distribution of CRISPR-Cas systems among the different bacterial and archaeal lineages, and will focus on our mechanistic understanding of the type I-E system of Escherichia coli. We will cover the overall organization of this system, starting with a detailed description of a typical type I-E gene cluster and its associated CRISPR array. In addition, we will describe recent insights on the three different stages in CRISPR-Cas type I-mediated defense: adaptation, expression and crRNA maturation, and interference. A comparison will be presented of the physical and functional characteristics of CRISPR effector complexes from the various subtype