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J Biol Chem. 2001 Jun 29;276(26):23581-8. Epub 2001 Apr 20.

Sau3AI, a monomeric type II restriction endonuclease that dimerizes on the DNA and thereby induces DNA loops.

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Institut für Biochemie, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany.


Here, we report that Sau3AI, an unusually large type II restriction enzyme with sequence homology to the mismatch repair protein MutH, is a monomeric enzyme as shown by gel filtration and ultracentrifugation. Structural similarities in the N- and C-terminal halves of the protein suggest that Sau3AI is a pseudo-dimer, i.e. a polypeptide with two similar domains. Since Sau3AI displays a nonlinear dependence of cleavage activity on enzyme concentration and a strong preference for substrates with two recognition sites over those with only one, it is likely that the functionally active form of Sau3AI is a dimer of a pseudo-dimer. Indeed, electron microscopy studies demonstrate that two distant recognition sites are brought together through DNA looping induced by the simultaneous binding of two Sau3AI molecules to the DNA. We suggest that the dimeric form of Sau3AI supplies two DNA-binding sites, one that is associated with the catalytic center and one that serves as an effector site.

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