Diverse evolutionary roots and mechanistic variations of the CRISPR-Cas systems

BACKGROUND Prokaryotes have evolved multiple systems to combat invaders such as viruses and plasmids. Examples of such defense systems include receptor masking, restriction-modification (R-M) systems, DNA interference (Argonaute), bacteriophage exclusion (BREX or PGL), and abortive infection, all of which act in an innate, nonspecific manner. In addition, prokaryotes have evolved adaptive, heritable immune systems: clustered regularly...