We classified interphase chromosome breaks into α and β forms to study the requirement for the xrs gene product in the repair of each of these forms of damage. The α form of damage comprises radiation-induced interphase chromosome breaks whose rejoining is slow and sensitive to treatment with β-arabinofuranosyladenine (β-araA), whereas the β form of damage comprises interphase chromosome breaks whose rejoining is fast and sensitive to treatment in hypertonic medium. Interphase chromosome breaks of the α form are visualized in plateau-phase cells by premature chromosome condensation (PCC) carried out in the absence of any treatment during the condensation period. More interphase chromosome breaks of the α form can be uncovered by treatment with β-araA during the period of PCC. Interphase chromosome breaks of the β form are not visualized in experiments using standard PCC protocols but can be uncovered by treatment in hypertonic growth medium during the period allowed for PCC. In the present report, we show that the yield of interphase chromosome breaks of the α form is similar in CHO and xrs-5 cells and demonstrate that xrs-5 cells rejoin this type of interphase chromosome breaks with an efficiency similar to that observed in repair-proficient CHO cells. Furthermore, we provide evidence supporting the notion that xrs-5 cells are deficient in the rejoining of the β form of interphase chromosome breaks. These results strongly suggest that the product of the xrs gene is required predominantly in the repair of the β form of interphase chromosome damage and emphasize the need for discrimination between different forms of interphase chromosome breaks in irradiated cells.

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