Escherichia coli Hfr-DNA degradation in endonuclease I-deficient minicells

[3H]Thymidine (dThd)-labelled Hfr DNA was transferred by conjugation into Escherichia coli F- minicells harvested from an endonuclease I-deficient (endI-) strain and its iosgenic wild type (endI+) parent. The susceptibility of this DNA to attack by DNAase was examined. The kinetics of in vivo conversion of [3H]dThd-labelled DNA into acid soluble radioactivity was examined. This activity, attributed to exonuclease action was the same for both strains. Contribution of endonuclease I was measured by an analysis of changes in weight-average (Mw) and number-average (Mn) molecular weight distribution of DNA molecules recovered from minicells. Reduction in Mw was greater in the endI-strain. The ratio Mn/Mw changed drastically during the incubation period of endI- minicells, but remained unchanged in the endI+ strain. These experiments suggest that the presence of the endI- mutation in minicell-producing strain chi1268 leads to a greater loss in M2 of Hfr DNA conjugally transferred into the minicells.