Type V CRISPR-Cas Systems

Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins are part of the bacterial and archaeal adaptive immunity that provides “memory” of past infection by foreign nucleic acids. This chapter addresses the evolutionary origins of the type V systems and highlighting the broad similarities and differences of the molecular mechanisms of adaptive immunity among their subtypes. Type V systems are classified based on the presence of a signature Cas12 effector that contains a characteristic RuvC nuclease domain at its C terminus. Type V systems can also be classified based on their RNA requirements. Viral fragments derive from processes that result in double-strand breaks and free DNA termini. CRISPR arrays, composed of alternating repeat and spacer segments, are transcribed as precursors that must be cleaved to produce mature crRNAs containing the sequence of a single spacer and can guide target binding.