Effect ofEscherichia coliIHF Mutations on Plasmid p15A Copy Number

Four isogenic strains (himAhimDdouble mutant,himAandhimDsingle mutants, and their wild type counterpart) harboringorip15A plasmid (pACYC184 or pACYC184Amp or pACYC177) show different copy numbers of that plasmid in the early stationary phase of cultivation. The copy number oforip15A plasmid increases about four times in thehimAhimDdouble (65–70 copies per cell) andhimDsingle mutant cells (50–56 copies per cell) and was almost the same inhimAmutant (17–18 copies per cell) and wild type cells (14–16 copies per cell). The results suggest that HimD can form homodimers, which are functionally competent for the regulation oforip15A plasmid copy number. Complementation experiments ofhimAhimDdouble mutant cells using plasmid carryinghimAandhimDgenes (pP_LhiphimA-5) confirm the effect of integration host factor (IHF) absence on increasing the copy number oforip15A plasmid (plasmid producing IHF complemented the defect of IHF mutant). The absence of IHF (usinghimAhimDdouble mutant as host) had no effect on the copy number of the pBR322 (oripMB1) plasmid.