Restriction endonucleases do induce sister-chromatid exchanges in Chinese hamster ovary cells

Restriction endonucleases (REs) AluI, EcoRI, MspI, PvuII and SmaI were introduced into exponentially growing Chinese hamster ovary (CHO) cells permeabilised by the bacterial cytotoxin, streptolysin O (SLO). Sister-chromatid exchanges (SCEs) were scored in posttreatment metaphases following recovery times of 20 and 24 h. At both fixation times, all five REs tested were found to increase the frequencies of SCEs compared with that of SLO-treated controls. Among the REs used, AluI and PvuII were found to be most effective, inducing on average a 2-3-fold increase, with a proportion of cells exhibiting more than 80 SCEs. These results confirm the earlier findings of Natarajan et al. (1985) and tend to suggest that double-strand breaks (DSBs) induced by REs are involved in the formation of SCEs.