Inactivation of mitotic factors by ultraviolet irradiation of HeLa cells in mitosis.

Extracts from mitotic HeLa cells, when injected into fully grown Xenopus laevis oocytes, exhibit maturation-promoting activity (MPA) indicated by germinal vesicle breakdown (GVBD) and chromosome condensation. Recently, we observed that the MPA of mitotic cell extracts is neutralized by the inhibitors of mitotic factors (IMF) in HeLa cells, which are activated at telophase and remain active throughout the G1 period. The activity of the IMF coincides with the process of chromosome decondensation, which begins at telophase and continues until the beginning of S phase, when chromatin reaches its most decondensed state. The objective of the present study was to investigate whether these two phenomena - chromosome decondensation and the activation of IMF - were related. The activity of IMF was measured in N2O-blocked mitotic HeLa cells, in which chromosome decondensation was induced by exposure to ultraviolet light, and subsequent incubation in medium containing inhibitors of DNA synthesis, hydroxyurea and arabinosylcytosine (araC). u.v. irradiation activated IMF was seen even at very high doses of X-irradiation. The IMF seemed to inactivate the mitotic factors directly by forming a complex that precipitated on heating at 60 degrees C for 15 min. Mg2+ or polyamines (i.e. spermine, spermidine, and putrescine), agents known to promote chromatin condensation partially restored the MPA of the u.v.-irradiated mitotic cell extracts. These results tend to support the conclusion that the IMF play a role in the decondensation of chromosomes.