Figure 3.

Nuclei stimulate MPF activation in vitro. (a) Histone H1 kinase activity during the first cell cycle of a Xenopus extract made 20 min after egg activation. Increasing volumes (0–8 μl) of nucleus suspension were added to 200 μl aliquots of the extract to give 0, 250, 1,000, and 4,000 nuclei/μl of extract, then divided into 10 μl aliquots, incubated at 21°C, and samples were taken at 5-min intervals over a 90-min period. 250 and 1,000 nuclei/μl advanced the peak of MPF activation by 10 min compared with extracts without nuclei. The delayed MPF activation in the presence of 4,000 nuclei/μl is explained by the activation of the DNA replication checkpoint (Dasso and Newport 1990). (b) Western blot performed with the same samples using anti-Cdc25C and anti-cyclin B2 antibodies. Maximum phosphorylation of Cdc25C and peak cyclin B2 coincided with peak H1 kinase activity.