Functional Domains in Fok I Restriction Endonuclease

The PCR was used to alter transcriptional and translational signals surrounding the Flavobacterium okeanokoites restriction endonuclease (fokIR) gene, so as to achieve high expression in Escherichia coli. By changing the ribosome-binding site sequence preceding the fokIR gene to match the consensus E. coli signal and by placing a positive retroregulator stem-loop sequence downstream of the gene, Fok I yield was increased to 5-8% of total cellular protein. Fok I was purified to homogeneity with phosphocellulose, DEAE-Sephadex, and gel chromatography, yielding 50 mg of pure Fok I endonuclease per liter of culture medium. The recognition and cleavage domains of Fok I were analyzed by trypsin digestion. Fok I in the absence of a DNA substrate cleaves into a 58-kDa carboxyl-terminal and 8-kDa amino-terminal fragment. The 58-kDa fragment does not bind the DNA substrate. Fok I in the presence of a DNA substrate cleaves into a 41-kDa amino-terminal fragment and a 25-kDa carboxyl-terminal fragment. On further digestion, the 41-kDa fragment degrades into 30-kDa amino-terminal and 11-kDa carboxyl-terminal fragments. The cleaved fragments both bind DNA substrates, as does the 41-kDa fragment. Gel-mobility-shift assays indicate that all the protein contacts necessary for the sequence-specific recognition of DNA substrates are encoded within the 41-kDa fragment. Thus, the 41-kDa amino-terminal fragment constitutes the Fok I recognition domain. The 25-kDa fragment, purified by using a DEAE-Sephadex column, cleaves nonspecifically both methylated (pACYCfokIM) and nonmethylated (pTZ19R) DNA substrates in the presence of MgCl2. Thus, the 25-kDa carboxyl-terminal fragment constitutes the Fok I cleavage domain.