Signal transaction between the functional domains of type II restriction endonuclease

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StsI is a member of unusual class of restriction enzymes that recognize a specific DNA sequence and cleave nonspecifically a short distance away from that sequence. The present study describes mutants of the restriction enzyme StsI that lose DNA cleavage activity. We constructed StsI variants, S233I, W241R, K300E, L365R, E442K, L453S, and I478T, which are unable to cleave DNA specifically. The variants, S233I and E442K, were purified homogeneously from the soluble fraction of recombinant E. coli cell, but the other mutant enzymes produced inclusion bodies. In order to purify these proteins from inclusion bodies, the aggregates were suspended in 6 M guanidine-HCl, and refolded by dilution with the appropriate buffer. The mutant proteins were purified homogeneously with subsequent chromatography using Heparin-Sepharose. The StsI variant, L365R, which defective for both DNA-binding and DNA-cleavage activities, stimulated the rate of DNA cleavage catalyzed by wild-type StsI. These results suggested that the StsI catalytic domains dimerize, as reported by FokI, but a second StsI molecule does not necessarily bind to the recognition site.In order to solve the crystal structure of StsI, an overexpression system was constructed and the enzyme was purified homogeneously. However, we have not been succeeded in obtaining the StsI crystal yet. Improvements have to be made in purification process of StsI.