Kinetics of integration of transforming DNA in pneumococcus

Abstract

Integration of donor genes, as measured by recovery of their transforming activity from eclipse in lysates of newly transformed cells of pneumococcus, has been followed at temperatures from 0 to 40 degrees . There is a lag in the recovery curve that is marker-dependent and increases as temperature falls. An Arrhenius plot of the rates shows a sharp break between 15 and 20 degrees . Brief exposure of the system to 37 degrees before incubation at 10 or 15 degrees removes the lag and raises the subsequent rate of recovery. This activation is unstable, however, and disappears when the cells are held at 0 degrees after the exposure at 37 degrees and before incubation at 15 degrees . The results are interpreted in terms of a reaction sequence A right harpoon over left harpoon B --> C, with activation energies for the first forward rate-constant of the order of 50 Cal/mol, for the second, 20-21 Cal/mol, and for the reverse reaction, less than 20 Cal/mol. The properties of the first step, including its marker dependence, are the same as those observed earlier for stabilization of donor markers against intracellular inactivation, and it is suggested they may reflect an activation of the recipient chromosome prerequisite to synapsis.