Exonuclease activity from Pseudomonas aeruginosa which is missing in phenotypically restrictionless mutants

A phenotypically restrictionless strain of Pseudomonas aeruginosa was found to lack a deoxyribonuclease specific for linear duplex DNA. The purified enzyme had an optimum pH of 8.5, required MgCl2 (10 mM) for maximum activity, and did not require ATP. Neither the degradation of heat-denatured DNA nor the degradation of bacteriophage F116 DNA was detected. The genome of bacteriophage F116 was shown to possess single-stranded terminal regions, which account for the resistance to degradation and for the ability of the phage to transfect restriction-proficient strains.