CRISPR-Cas Systems and Anti-CRISPR Proteins: Adaptive Defense and Counter-Defense in Prokaryotes and Their Viruses

For billions of years, prokaryotes and their viruses, known as phages, have been engaged in an antagonistic co-evolutionary arms race that has given rise to an impressive arsenal of defensive and offensive molecular weaponry. CRISPR-Cas is a class of adaptive immune systems that employ small RNAs and CRISPR-associated (Cas) nucleases to identify and destroy foreign nucleic acids such as those originating from phage. Phages, in turn, have evolved diverse anti-CRISPR proteins which undermine CRISPR-Cas function. Here, we review the discoveries and mechanisms of CRISPR-Cas systems and anti-CRISPR proteins, two remarkable molecular innovations that have emerged from the phage-host arms race.