Characterization of homologous recombination induced by replication inhibition in mammalian cells

To analyze relationships between replication and homologous recombination i n mammalian cells, we used replication inhibitors to treat mouse and hamste r cell lines containing tandem repeat recombination substrates. In the firs t step, few double-strand breaks (DSBs) are produced, recombination is slig htly increased, but cell lines defective in non-homologous end-joining (NHE J) affected in ku86 (xrs6) or xrcc4 (XR-1) genes show enhanced sensitivity to replication inhibitors. In the second step, replication inhibition leads to coordinated kinetics of DSB accumulation, Rad51 foci formation and RAD5 1-dependent gene conversion stimulation. In xrs6 as well as XR-1 cell lines , Rad51 foci accumulate more rapidly compared with their respective control s. We propose that replication inhibition produces DSBs, which are first pr ocessed by the NHEJ; then, following DSB accumulation, RAD51 recombination can act.