Restriction endonucleases and their recognition sequences.

Any survey of Type-I1 restriction endonucleases requires a consideration of the various specificities that are the hallmark of these enzymes. The latest compilation of the restriction-enzyme list contains more than 200 enzymes, which exhibit more than 50 unique specificities (Roberts, 1980). Because these enzymes have demonstrated their greatest value as biochemical tools, classification of these enzymes into groups based on common themes inherent within their varied specificities provides an attractive scaffolding for this presentation. The unique specificities of the currently known restriction endonucleases are listed in Table 1 based on the size of their recognition sites.* These recognition sequences vary in length from four to six base pairs. An important addition to the tetranucleotide group of enzymes has been made with the recent discovery of enzyme RsaI (GTAC) by J. F. Gardner & S. Kaplan (unpublished work). With the description of this recognition site, eight of the 16 possible tetranucleotide palindromic combinations have now been discovered. Another restriction endonuclease of interest listed within the tetra-