FIGURE 3.

Extein dependence. A, in the DnaE Ssp DnaE intein, Phe+2 packs against Block F histidine, modulating its pose and hence increasing efficiency. B, DnaB Ssp mini-intein can only tolerate Gly at −1 position; any bigger residue (i.e. the alanine shown) creates steric clashes in the active site. C, the native −1 amino acid in the Pho RadA intein (PDB 4E2T) is a lysine that makes charge-charge interaction with Glu⁷¹ (Lys−1 was modeled by using CHIMERA (49)). D, −1 residue in Pho RadA mini-intein tightly interacts with Glu⁷¹, Val⁷³, and Val¹⁵¹ (PDB 4E2U).