CRISPR-Mediated Adaptive Immune Systems in Bacteria and Archaea.

Effective clearance of an infection requires that die immune system rapidly detects and neutralizes invading parasites while stricdy avoid-ing self-antigens that would result in autoimmunity. The cellular ma-chinery and complex signaling pathways diat coordinate an effective immune response have generally been considered properties of the eu-karyotic immune system. However, a surprisingly sophisticated adap-tive immune system that relies on small RNAs for sequence-specific targeting of foreign nucleic acids was recently discovered in bacteria and archaea. Molecular vaccination in prokaryotes is achieved by inte-grating short fragments of foreign nucleic acids into a repetitive locus in the host chromosome known as a CRISPR (clustered regularly in-terspaced short palindromic repeat). Here we review the mechanisms of CRISPR-mediated immunity and discuss the ecological and evolu-tionary implications of these adaptive defense systems. [ABSTRACT FROM AUTHOR]