Transformation Frequency of Gamma-Irradiated Plasmid DNA and the Enzymatic Double Strand Break Formation by Incubation in a Protein Extract of Escherichia Coli

Abstract

It was found that incubation of ?-irradiated or DNaseI-treated plasmid DNA in a protein extract of Escherichia coli leads to enzyme-induced formation of double strand breaks (dsb) in competition with repair of precursors of these dsb. A survival curve of the plasmid DNA (as determined by transformation of E. coli was calculated on the basis of enzyme-induced dsb as well as those produced by irradiation assuming that they are lethal. The calculated D0 value was the same as that measured directly by transformation of irradiated plasmid DNA. Two models are presented that fit the experimental survival data as a function of dose. One is based on damage formation in the plasmid DNA including enzymatic conversion of single strand damage into dsb (U-model), the other is an enzymatic repair saturation model based on Michaelis-Menten kinetics. A closer inspection of the models shows that the U-model does not incorporate the observed repair saturation and the repair saturation model does not incorporate the observed higher order formation of dsb. A more valid model may be developed by combining the two models.