Special focus CRISPR-Cas

Six years ago a new sophisticated prokaryotic defense system was identified that was termed CRISPR-Cas (CRISPR, clustered regularly interspaced short palindromic repeats; Cas, CRISPR associated). Parts of it, the mysterious repeat sequences, were already detected in 1987 but their function remained unknown for the next 20 y. The CRISPR-Cas system is an adaptive, heritable defense system that prokaryotes use to protect themselves against invaders such as viruses. Three highly diverse types of CRISPR-Cas that display major functional and structural differences exist (type I–III); and these three major types have been further divided into 10 subtypes, that also show considerable differences (subtypes IA-IF, IIA-B, IIIA-B). The key elements of these defense systems are the Cas proteins and the CRISPR RNA. The CRISPR RNA consists of short repeat sequences interspersed with spacer sequences derived from invader DNA. Since its discovery, much has been learned about this system: we now know that there are a variety of CRISPR-Cas subtypes and that the Cas proteins may have multiple functions. Biotechnological applications were implicated early after the discovery of this system, and a new avenue of potential applications was recently reported. However, our understanding of this system is far from complete. In this special issue, the state of this system is summarized in three reviews and three commentaries, and new data are reported in 19 research articles. The evolution of this system and its components is described in the review by Koonin et al. They describe an interesting connection between the CRISPR-Cas system and the toxin-antitoxin systems.