Host-controlled restriction of T-even bacteriophages: relation of four bacterial deoxyribonucleases to restriction.

Escherichia coli strains B and K-12, which restrict growth of nonglucosylated T- even phage (T(*) phage), and nonrestricting strains (Shigella sonnei and mutants of E. coli B) were tested for levels of endonuclease I and exonucleases I, II, and III, by means of in vitro assyas. Cell-free extracts freed from deoxyribonucleic acid (DNA) were examined with three substrates: E. coli DNA, T(*)2 DNA, and T2 DNA. Both restricting and nonrestricting strains had comparable levels of the four nuclease activities and had similar patterns of preference for the three substrates. In addition, mutants of E. coli B and K-12 that lack endonuclease I were as effective as their respective wild types in restricting T(*) phage.