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Table 1 Experimentally characterized and predicted functions of the core components of CRISPR-Cas systems

From: Unification of Cas protein families and a simple scenario for the origin and evolution of CRISPR-Cas systems

Family Experimental/in silicoevidence Prediction
Cas1 Metal-dependent deoxyribonuclease; a unique fold consisting of a N-terminal β strand domain and a C-terminal α-helical domain [21, 68]; also binds RNA [21, 68] Involved in integration of spacer DNA into CRISPR repeats.
Cas2 RNAse specific to U-rich regions [23] Facilitates spacer selection and/or integration. Could be involved in further crRNA cleavage.
Cas3 (helicase and HD domain) Single-stranded DNA nuclease (HD domain) and ATP-dependent helicase [24]; required for interference [13]. Cuts DNA during interference; promotes strand separation.
Stand alone HD nuclease Metal-dependent deoxyribonuclease specific for double-stranded oligonucleotides [69]. Cuts DNA during interference.
Cas4 RecB-like nuclease homolog with three-cysteine C-terminal cluster [5] Might be involved in spacer acquisition
Cas5 RAMP [5], subunit of Cascade complex [8, 13]. Might substitute for Cas6 if catalytically active. Otherwise might be involved in both interference and adaptation stages.
Cas6 RAMP [5]; metal-independent endoribonuclease that generates crRNAs, subunit of Cascade complex [8, 1315, 17].  
Cas7 RAMP [5], subunit of Cascade complex [13]; present Cascade complex of I-E systems in 6 copies [8] and in several copies in I-A systems [16]. Implicated in interference; binds crRNA; if enzymatically active, might be involved in RNA-guided RNA cleavage.
Cas8 (large subunit) Subunit of Cascade complex [13]. Inactivated Cas10 polymerase-like protein, binds DNA, interacts with HD domain and a RAMP carrying crRNA; could be involved in both interference and spacer selection stages.
Cas10 (large subunit, CRISPR polymerase) Subunit of Cascade (Cmr) complex [15]; homologous to Palm domain polymerases and cyclases. Same as Cas8, but fused to HD and thus cuts ssDNA; might be involved in strand separation.
Small subunit Small, mostly alpha helical protein, subunit of Cascade complex [13, 15]; present in Cascade complex of I-E systems in two copies [8] Specifically binds DNA; might recognize PAM.