In maturing mouse oocytes, p34cdc2-associated histone H 1 kinase activity gradually increases until it reaches its maximum at metaphase I (Choi et al., 1991: Development 113:789-795). In this study, treatment of oocytes with cycloheximide resulted in a failure to increase the level of histone H1 activity above that detected at approximately the time of germinal vesicle breakdown (GVB), which is -20-30% of the level normally achieved at metaphase I. Cyclin B was detected in GVstage oocytes, but there was a 2-2.5-fold increase in the amount of cyclin B in maturing oocytes from GV-stage to metaphase I and a burst of cyclin B synthesis during the first 3 hr of maturation. Okadaic acid-treatment of mouse oocytes did not accelerate activation of histone H1 kinase but rather arrested its activity at the same level observed in cycloheximide-treated oocytes. Thus the components of the ~3 4 " ~~~ kinase activating system in mouse oocytes are apparently not present in GV-stage oocytes in an amount or configuration that would allow maximum kinase activation when meiosis is reinitiated by okadaic acid. Importantly, okadaic acid-treatment dramatically inhibited protein synthesis. Therefore, the inhibition of protein synthesis by okadaic acid probably abrogates the possibility of de novo synthesis of the regulators of ~3 4 ~~~' kinase required to drive its activity to the maximum level normally achieved by metaphase 1. It is concluded that there is a critical point in driving the continued activation of histone H1 kinase that occurs at approximately the time of GVB.
Progression beyond this point requires de novo protein synthesis. Since newly synthesized cyclin B is immediately complexed with the p34cdc2 kinase in maturing mouse oocytes, cyclin B is a candidate for one of the proteins whose synthesis is required to drive the continued increase in histone H1 kinase activity after the time of GVB.