SEQUENCE SIMILARITY AMONG TYPE-II RESTRICTION ENDONUCLEASES, RELATED BY THEIR RECOGNIZED 6-BP TARGET AND TETRANUCLEOTIDE-OVERHANG CLEAVAGE

The type-II restriction endonucleases (ENases) EcoRI (recognition sequ ence G down arrow AATTC), RsrI (G down arrow AATTC), XcyI (C down arro w CCGGG), Cfr9I (C down arrow CCGGG) and MunI (C down arrow AATTG), al l cleave hexanucleotide palindromic sequences, leaving tetranucleotide 5'-overhangs. Two regions of similarity that appear in the same order and relative position were identified among the amino-acid sequences of ENases. These regions map to the structural elements of EcoRI invol ved in the building of the catalytic site and in interactions with the central nucleotides of the recognized sequence. We propose that these ENases might all share a similar structural organization of the activ e site and structural motifs involved in interactions with specific DN A recognition sequences.