Repair of DNA single-strand breaks in X-irradiated yeast. II. Kinetics of repair as measured by the DNA-unwinding method.

The kinetics of disappearance of single-strand breaks (SSB) from the DNA of X-irradiated stationary yeast cells under liquid-holding conditions was found to proceed in a dose-independent manner up to a dose of at least 2400 Gy, and was found to be complete after incubation of cells for 1 h. This was deduced from data for a yeast wild-type (WT) haploid and diploid strain as well as for rad52 haploid cells defective in DNA double-strand break (DSB) repair. In all cases an initial fast repair component assumed to correspond to SSB repair was observed whereby about 80% of the induced 'unwinding points' disappeared from the DNA with a time constant of about 3 min. Following this fast component, a slower component of removal of 'unwinding points' occurred with a time constant estimated to be 20 min. The molecular nature of these two components of repair is not known. We could find no evidence for the induction of secondary (enzymatic) breaks in the DNA during post-irradiation incubation. Incubation of cells in growth medium after irradiation resulted in similar kinetics as those under liquid-holding conditions. In the absence of an energy source in the medium (i.e. when cells were incubated in buffer or distilled water after irradiation) only 60-80% of the SSB were removed from yeast DNA. Residual SSB disappeared from the DNA only when cells were transferred to a medium containing glucose. The relative mass of DNA unwound per induced strand break (i.e. represented by the slope of the dose-effect curve immediately after irradiation) was found to change slowly with the age of the cell culture under liquid-holding conditions. This effect had to be corrected for in the measurements of strand break repair under these conditions.