Figure 1.—

Screen strategy. (1) The rad9Δ/rad9Δ strain ES75 was mutagenized by transformation with the NotI-digested mTn-lacz/Leu2 insertional library and positive transformants were selected on leucine-deficient plates. (2) Approximately 185 colonies from each mutant strain were then struck out on low-adenine media and monitored for sectoring colonies as a measure of CF loss. Strains that met the sectoring threshold were restruck three times and strains that consistently showed increased CF loss were carried forward. (3) The transposon insertion site was then identified in mutagenized strains that showed increased CF loss. (4) The rate of chromosome V loss or recombination as quantitatively measured by conversion to canavanine resistance was estimated using fluctuation analysis for those strains that demonstrated increased CF loss.