The position of the six RNA polymerase intein insertion sites mapped onto the crystal structure of the RNA polymerase II of Saccharomyces cerevisiae. Representation of S. cerevisiae RNA polymerase II (PDB: 1I3Q); the second largest subunit is coloured dark grey (other subunits, including the largest, are coloured light brown). Top: surface view showing the position of the cleft formed by the two largest subunits and the position of the four intein insertion sites (indicated by different colours) on the surface of the cleft, near the active site/"wall" region. Lower: three surface views from a different orientation; the middle image has all of the subunits other than the second largest subunit as a semi-transparent surface so that the position of the two intein insertion sites on the interface between RPB1 and RPB2 (red and blue regions) can be seen. The lowest image is of RPB2 only, but with the intein insertion sites labelled with the names of inteins inserted at these positions in some homologues.