The DNA cleavage produced by 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) in mammalian cells is putatively mediated by topoisomerase II. We found that in synchronized HeLa cells the frequency of such cleavage was 4-15-fold greater in mitosis than in S while the DNA of G1 and G2 cells exhibited an intermediate susceptibility to cleavage. The hypersensitivity of mitotic DNA to m-AMSA-induced cleavage was acquired relatively abruptly in late G2 and was lost similarly abruptly in early G1. The susceptibility of mitotic cells to m-AMSA-induced DNA cleavage was not clearly paralleled by an increase in topoisomerase II activity (decatenation of kinetoplast DNA) in 350 mM NaCl extracts from mitotic cells compared to similar extracts from cells in G1, S, or G2. Furthermore, equal amounts of decatenating activity from cells in mitosis and S produced equal amounts of m-AMSA-induced cleavage of simian virus 40 (SV40) DNA; i.e., the interaction between m-AMSA and extractable enzyme was similar in mitosis and S. The DNA of mitotic cells was also hypersensitive to cleavage by 4'-demethylepipodophyllotoxin 4-(4,6-O-ethylidene-beta-D-glucopyranoside) (etoposide), a drug that produces topoisomerase II mediated DNA cleavage without binding to DNA. Thus, alterations in the drug-chromatin interaction during the cell cycle seem an unlikely explanation for results in whole cells. Cell cycle stage dependent fluctuations in m-AMSA-induced DNA cleavage may result from fluctuations in the structure of chromatin per se that occur during the cell cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Our earlier studies indicated that the mitotic factors, which induce germinal vesicle breakdown and chromosome condensation when injected into fully grown Xenopus oocytes, are preferentially associated with metaphase chromosomes and that they bind to chromatin as soon as they are synthesized during the G2 phase . In this study, we attempted to determine the fate of these factors as the cell completes mitosis and enters G, . Extracts from HeLa cells at different points during G,, S, and G2 periods were mixed with mitotic extracts in various proportions, incubated, and then injected into Xenopus oocytes to determine their maturationpromoting activity. The maturation-promoting activity of the mitotic extracts was neutralized by extracts of G, cells during all stages of G, but not by those of late S and G 2 phase cells . Extracts of quiescent (Go) human diploid fibroblasts exhibited very little inhibitory activity . However, UV irradiation of G o cells, which is known to cause decondensation of chromatin, significantly enhanced the inhibitory activity of extracts of these cells. These factors are termed inhibitors of mitotic factors (IMF) . They seem to be activated, rather than newly synthesized, as the cell enters telophase when chromosomes begin to decondense. The IMF are nondialyzable, nonhistone proteins with a molecular weight of >12,000 . Since mitotic factors are known to induce chromosome condensation, it is possible that IMF, which are antagonistic to mitotic factors, may serve the reverse function of the mitotic factors, i.e ., regulation of chromosome decondensation .Chromatin undergoes profound structural alterations during the cell cycle . During mitosis, it is supercoiled and condensed to form the mitotic chromosomes. Chromosome condensation is a critical event in the cell cycle that is necessary for the equal distribution of genetic material between the daughter cells. The phenomenon of premature chromosome condensation described by Johnson and Rao (13) has been very useful in visualizing the state of chromatin condensation during interphase. Using this technique, Rao and Hanks (30) and Hanks and Rao (9) have shown that the process of chromosome decondensation, initiated during the telophase of mitosis, continues throughout the G, period until the chromatin reaches its ultimate state ofdecondensation by the end of G,, when DNA synthesis is initiated .As for the factors involved in chromosome condensation, studies from this and other laboratories have demonstrated that when extracts from mitotic HeLa cells (39,40) or mitotic Chinese hamster ovary cells (19) are injected into Xenopus THE JOURNAL OF CELL BIOLOGY -VOLUME 97 DECEMBER 1983 1707-1713 C The Rockefeller University Press -0021-9525/83/12/1707/07 $1 .00 laevis oocytes, they exhibit maturation-promoting activity (MPA)' as evidenced by germinal vesicle breakdown (GVBD) and condensation of chromosomes. These studies also revealed that the mitotic factors are nonhistone proteins and accumulate slowly in the beginning of G2, increase rapid...

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