In vivo restriction. Sequence and structure of endonuclease II-dependent cleavage sites in bacteriophage T4 DNA.

Endonuclease II of bacteriophage T4 is required for in vivo restriction of cytosine-containing DNA from its host, Escherichia coli, (as well as from phage mutants lacking cytosine modification), normally the first step in the reutilization of host DNA nucleotides for synthesis of phage DNA in infected cells. The phage cytosine-DNA is fragmented incompletely to yield genetically defined fragments. This restriction is different from that of type I, II, or III restriction enzymes. We have located seven major endonuclease II-dependent restriction sites in the T4 genome, of which three were analyzed in detail; in addition, abundant sites were cleaved in less than or equal to 5% of all molecules. Sites I, II, and III shared the sequence 5'-CCGNNTTGGC-3' and were cleaved in about 25% (I and III) and 65% (II) of all molecules, predominantly staggered around the first or second of the central unspecified base pairs to yield fragments with one 5' base. The less frequently cleaved sites I and III deviated from site II in predicted helical structure when viewed from the consensus strand, and in sequence when viewed from the opposite strand. Thus, interaction with a particular helical structure as well as recognition of the bases in DNA appears important for efficient cleavage.