ENDONUCLEASE-II OF COLIPHAGE T4 - A RECOMBINASE DISGUISED AS A RESTRICTION-ENDONUCLEASE

Endoll shares with restriction endonucleases the property of cleaving foreign DNA while leaving the endonuclease-encoding genome intact, ens uring the survival of one DNA species in the cell. In addition, in viv o Endoll cleaves a specific DNA sequence and cleavage is context depen dent. These context effects extend over at least 1000 bp, largely limi ting cleavage to once within this distance. Like homing endonucleases, in vivo Endoll recognizes a long, asymmetric and degenerate consensus sequence which has two distinct parts. Recognition of one part of the consensus sequence involves base-specific bonds, and recognition of t he other involves sequence-dependent helical structure. Endoll fulfils an obvious short-term survival role in ensuring the dominance of phag e DNA in an infected cell, but may also have a long-term evolutionary role, producing gene-size fragments of foreign DNA to be enrolled in t he phage genetic repertoire.