(A) Insertion contexts of ΔI-SM::CBD in I-TevI relative to I-TevI structure. Cysteines before which ΔI-SM::CBD was inserted are presented in space-filling configuration in the context of a composite I-TevI ribbon diagram derived from the independently solved crystal structures of the N- and C-terminal domains of the enzyme (residues 1–97 and 130–245, respectively) (23,24). Residues between 93 and 147 are likely unstructured or loosely structured. (B) Properties of fusion derivatives. Viability of pTev::ΔI-SM::CBD fusions are denoted as follows: =, lethality in non-expression strain JM101; –, + and ++, no growth, slow growth and normal growth, respectively, in BL21(DE3)pLysS. The approximate in vivo distribution of precursors and reaction products for all seven MBP-TevR27A:: ΔI-SM::CBD constructs is shown. (See also Fig. 3B.) For fusions at C207 and C214, the percentages for precursor and C-extein were not independently quantifiable and were combined (brackets). LE+N-ex, combined ligated exteins and N-terminal extein (bold).