Possible interactions of C-REase fusion polypeptides. A. Unfused systems such as PvuII, where the C protein and REase form separate homodimers. B. Fused system in which the linker between C and REase regions of the polypeptide is long enough and flexible enough to allow simultaneous dimerization at both C and REase subunit interfaces. C. Fused system in which dimerization of the C portion is in competition with dimerization of the REase portion. D. Fused system in which concatameric chains can form. See text for details.