DNA topoisomerase II activity in nonreplicating, transcriptionally inactive, chicken late spermatids.

To study a possible differential involvement of type I and type II DNA topoisomerases in the functional and structural changes that chromatin undergoes during spermatogenesis, we have determined both enzymatic activities in chicken testis cell nuclei at successive stages of differentiation. Whereas DNA topoisomerase I varies in parallel with transcriptional activity, DNA topoisomerase II was present in both replicating, transcriptionally active chicken testis cells and nonreplicating, transcriptionally inactive late spermatids. The presence of DNA topoisomerase II activity in late spermatids and, in addition, the relative increment of drug-induced topo-II-mediated DNA cleavage detected in these cells, suggest that DNA topoisomerase II might modulate the topology of DNA during the marked changes that chromatin structure undergoes in the nucleohistone-nucleoprotamine transition at the end of the spermiogenesis and could be involved in the final organization of DNA within the nucleus of the male gamete.