 The shorter S-peptide piece is behind, starting at upper left with a helix and ending with the chain break (between residues 20-21) at lower right.The active site for RNA cleavage (in the groove at center front in this drawing) involves one histidine side chain from the S-peptide fragment and another from the S-protein part.The computer image shows superimposed structures of ribonuclease S and A, with the S-peptide in gold and the active site histidines in hot pink.