Fig. 3. MID-PIWI and the APAZ domain grip the gRNA/tDNA in the catalytically active SPARTA oligomer.

a The gRNA/tDNA duplex as a whole is accommodated in a channel that runs between the MID-PIWI and the APAZ domains. DNA is shown in light grey and the cartoon representation of MID-PIWI and APAZ domains are shown in blue and green, respectively. Key residues interacting with both the gRNA and tDNA strands as well as Mg²⁺ ion are highlighted. b Schematic representation of the interactions of MID-PIWI and APAZ domains with gRNA/tDNA strands. Amino acid residues of the MID-PIWI and APAZ domain are indicated by blue and green colors, respectively. DNA backbone phosphates are shown as red circles. Hydrogen bonds are indicated by purple lines (determined by interaction distance <3.6 Å), electrostatic interactions are shown by orange lines and van der waals interactions are shown by black bars. All the bases constituting the gRNA and tDNA strands used for the reconstitution of the SPARTA complex are shown at the bottom. The modeled RNA, DNA and unmodeled bases in the catalytically active SPARTA oligomer are shown in red, blue and black letters, respectively.