Endonuclease II of coliphage T4: a recombinase disguised as a restriction endonuclease?

EndoII shares with restriction endonucleases the property of cleaving foreign DNA while leaving the endonuclease-encoding genome intact, ensuring the survival of one DNA species in the cell. In addition, in vivo EndoII cleaves a specific DNA sequence and cleavage is context dependent. These context effects extend over at least 1000 bp, largely limiting cleavage to once within this distance. Like homing endonucleases, in vivo EndoII 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 the other involves sequence-dependent helical structure. EndoII fulfills an obvious short-term survival role in ensuring the dominance of phage 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 the phage genetic repertoire.