Poly (ADP-ribosylation) and DNA topoisomerase I in different cell lines

The modification of DNA topoisomerase I activity by poly (ADP-ribosylation) is a potentially important control mechanism for this ubiquitous enzyme which is widely believed to play a role in replication (2, 3). Our studies of the phenomenon so far suggest that DNA topoisomerase I activity may be regulated in response to the state of the DNA in the nucleus since the modification enzyme, poly (ADP-ribose) synthetase, is sensitive to interruptions in the normal double helical structure of DNA (10). Although this phenomenon has been most extensively characterized for the purified enzymes from calf thymus (5), preliminary evidence for poly (ADP-ribosylation) of topoisomerase I is presented in a nonthymic system, the mammalian cell line CHO EM9. Attempts to detect poly (ADP-ribosylation) in yeast cells have so far proven to be unsuccessful.