FIG. 4.

Determination of SSBs by S1 nuclease treatment. MMS treatment of chromosomal DNA of WT yeast within plugs, along with the presence or absence of S1 nuclease, is indicated. DNA plugs were incubated in buffer either at 4°C (no heat treatment; lanes 1 to 10) or at 55°C (heat treatment) for 24 h to convert heat-labile sites into SSBs (lanes 11 to 20). Thereafter, S1 nuclease (4 U/ml for 20 min) treatment was applied to both mock heat-treated (lanes 6 to 10) and heat-treated (lanes 16 to 20) plugs.