TRANSFORMATION FREQUENCY OF GAMMA-IRRADIATED PLASMID DNA AND THE ENZYMATIC DOUBLE-STRAND BREAK FORMATION BY INCUBATION IN A PROTEIN EXTRACTOF ESCHERICHIA-COLI

It was found that incubation of gamma-irradiated or DNaseI-treated pla smid DNA in a protein extract of Escherichia coli leads to enzyme-indu ced formation of double strand breaks (dsb) in competition with repair of precursors of these dsb. A survival curve of the plasmid DNA (as d etermined 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 mea sured directly by transformation of irradiated plasmid DNA. Two models are presented that fit the experimental survival data as a function o f dose. One is based on damage formation in the plasmid DNA including enzymatic conversion of single strand damage into dsb (U-model), the o ther is an enzymatic repair saturation model based on Michaelis-Menten kinetics. A closer inspection of the models shows that the U-model do es not incorporate the observed repair saturation and the repair satur ation model does not incorporate the observed higher order formation o f dsb. A more valid model may be developed by combining the two models .