Type II restriction endonucleases are enzymes that have a high specificity and are of a great variety. In general, these enzymes are dimeric and recognize DNA sequences that vary between four and eight base pairs, and require only Mg2+ as a cofactor to catalyse the hydrolysis of DNA. To cleave another site, the enzyme has to first dissociate and rebind at another recognition site. However, a growing number of restriction enzymes (REases) have now been shown to bind to two DNA sites simultaneously. These enzymes fall into different subclasses which includes type IIF enzymes such as SfiI which is tetrameric and cleaves both DNA sites concertedly. It is the best biochemically and functionally characterised endonuclease amongst the other IIF enzymes. SfiI is also one of the few REases with an eight base-pair palindromic DNA recognition sequence.

This entry represents a superfamily that has the REase alpha/beta core comprised of a central twisted beta sheet surrounded by four alpha helices. The first three strands of the central beta-sheet are antiparallel and form a beta-meander that carry the catalytic residues at one end. The other end of the beta-meander is atypically associated with the SfiI tetramer interface.

PMID:16308566