Chromosome damage induced by restriction endonucleases recognizing thymine-rich DNA sequences in electroporated CHO cells.

The influence of BrdU substitution of DNA in Chinese hamster cells on the frequencies of chromosomal aberrations induced by three restriction endonucleases which recognize thymine-rich sequences in DNA has been studied. The restriction enzymes chosen were Eco RI (recognition site G/AATTC), Sca I (AGT/ACT), and Dra I (TTT/AAA). A restriction enzyme that does not have thymine in the recognition sequence, Hae III (GG/CC), was also tried. These enzymes were introduced into cells by electroporation after two cell cycles of BrdU substitution and the aberration yields compared with that observed in non-substituted cells. Our results seem to indicate that the BrdU-substituted chromatin becomes resistant to the chromosome-breaking activity of the restriction enzymes recognizing thymine-rich DNA sequences. These observations are compared with the patterns of cutting of isolated DNA as shown by agarose gel electrophoresis.