DNA DOUBLE-STRAND BREAK REJOINING IN XRS5 CELLS IS MORE RAPID IN THE G2 THAN IN THE G1 PHASE OF THE CELL-CYCLE

The radiosensitive xrs5 mutant cell line of CHO K1 shows an overall de ficiency in DNA double-strand break (dsb) rejoining. However, xrs5 par adoxically shows an apparently normal rate of disappearance of chromat id breaks with time, the kinetics of which is thought to reflect the u nderlying rejoining of dsb. Nevertheless the yield of chromatid breaks is elevated by four-fold in xrs5. A possible explanation of the parad ox might be that xrs5 is proficient in rejoining dsb in the G2 phase o f the cell cycle but converts a higher number of dsb into chromatid br eaks. In order to test this we have measured the rejoining df dsb in p artially synchronised G2 xrs5 cells and compared the kinetics with tho se of cells synchronised in the G1 phase. Synchronisation of cells was achieved in G2 by release of cells from an aphidicolin block, and in G1 by staurosporine block. Cell synchrony was monitored by cytofluorom etry and showed typically a 67% synchronisation of G2 cells and a 91% synchronisation of G1 cells. Rejoining of dsb was measured using neutr al filter elution at pH 9.6. G2 cells showed a two-component kinetic w ithvalues for G1 cells were 15 min and approximately 8.8 h. The t(1/2) values of 9 min and 3.6 h for dsb rejoining. Corresponding t(1/2) t(1 /2) value of 3.6 h found for dsb rejoining in G2 cells is similar to a previously published value for asynchronous parental CHO K1 cells of approximately 4 h. The kinetics of chromatid break rejoining was measu red in both xrs5 and CHO K1 following a dose of 0.75 Gy. The kinetics were found to be similar (t(1/2) = 2.4 h) in the two cell lines, as pr eviously reported using an equiclastogenic dose.