Structure of the active site of ribonuclease T, illustrating the conformational changes responsible for substrate selectivity against cytosine residues. The enzyme bound to DNA with a 3' cytosine (white) is in an inactive conformation, while the enzyme bound to DNA with a 3' guanosine (blue) is in an active conformation. Rendered using PyMol from PDB ID 3NH1 (active, blue) and PDB ID 3NGZ (inactive, white). Both structures from Hsaio et al. Structural basis for RNA trimming by RNase T in stable RNA 3'-end maturation. Nat Chem Biol 2011, PMID 21317904