Repair of DNA single-strand breaks in X-irradiated yeast. I. Use of the DNA-unwinding method to measure DNA strand breaks.

The DNA-unwinding method developed by Ahnström and his coworkers to measure DNA strand breaks in mammalian cells was used to measure single-strand breaks (SSB) in the DNA of intact yeast cells. DNA unwinding, which took place inside the rigid cell wall of yeast, was investigated as a function of time, radiation dose, and of pH and salt concentration of the alkaline solution. After DNA unwinding had taken place, the cell wall was destroyed by partial enzymatic digestion and sonication in the presence of detergents. Fragments of single- and double-stranded DNA were separated using hydroxylapatite chromatography. In this way the most suitable conditions for DNA unwinding within the cell wall were established. The results show that SSB and double-strand breaks (DSB) give rise to different kinetics of DNA unwinding.