Table 1.

Comparison of three types of CRISPR-Cas systems

	Type-I	Type-II	Type-III
CRISPR-cas action
Adaptation
Whether to depend on PAM when selecting proto-spacers	Yes	Yes	No
Expression
Pre-crRNA conjugates	Cascade complex	Cas9 (Csn1/Csx12) tracrRNA	Cas6 Csm (subtype III-A)/ Cmr (subtype III-B)a
Pre-crRNA cleavage enzymes	Cas6e subunit (subtype I-E)/Cas6f subunit (subtype I-F)	Housekeeping RNase III	Cas6
Processes to mature crRNA	1. A typical 8-nucleotide repeat fragment on the 5′ end 2. A hairpin structure on the 3′ flank	Cleavage at a fixed distance within the spacers (probably catalyzed by Cas9)	1. Cas6 is responsible for the processing step 2. Trimming the 3′ end of the crRNA further (Nucleases have not yet been identified.)
Interference
Methods of target recognition	Cascade complex guided by crRNA	Cas9 loaded with crRNA directly	The invading DNA fragment having no base pairing to the 5′ repeat fragment of the mature crRNA (resulting in interference)
Targets cleavage enzymes	Cas3	Cas9	Cas6 or Cmr/Csm complex
Targets	DNA	DNA	DNA (III-A)/ RNA (III-B)
Whether to depend on PAM to cleaving process	Yes	Yes	No
Special systems contained	cas3 gene	‘HNH’-type system	Polymerase and RAMP modules
Distribution of the three types of CRISPR-Cas systems	More common in Archaea	Only in Bacteria	More common in Archaea

^aThe pre-crRNA is transferred to a distinct Cas complex after Cas6 processing