Structure of the Cas1/Cas2 complex (PDB code 4P6I). Cas1 protomers (labelled as in ) are shown in gold and orange, the central Cas2 dimer in blue and green. (A) Arrows show the active sites (marked by His208, shown as blue sticks) of the two Cas1 protomers that bind directly to the Cas2 dimer. The distance between the active sites, which point away from each other, is approximately 81 Å. (B) Horizontal rotation of the view of the complex assembly reveals that the active sites of the other two Cas1 protomers in the heterohexamer are more convincingly pointed towards each other (as expected if these active sites mediate coordinated integration of a protospacer) and are separated by approximately 93 Å. However, the as-the-crow-flies connection between these two active sites passes directly through the intervening Cas2 dimer.