Growth condition-induced precocious activation of p34cdc2 kinase inhibits the expression of developmental competence.

We examined the effect of growth conditions upon mitotic p34cdc2 kinase and developmental competence in Acanthamoeba castellanii. At G2/M of the cell cycle p34cdc2 kinase activity peaks in level, and p34cdc2 is transiently in a complex with newly synthesized cyclin B and phosphorylated on tyrosine (pre-MPF). Developmental competence peaks in level slightly preceding p34cdc2 kinase activation and pre-MPF accumulation. Under adverse growth conditions p34cdc2 kinase activation and transient pre-MPF accumulation occur at a reduced G2 phase length and smaller cell size. Developmental competence is not expressed during the shortened G2 phase. Cycloheximide inhibits the precocious p34cdc2 kinase activation and both the precocious accumulation of cyclin B and tyrosine phosphorylation on p34cdc2. No-codazole inhibits the precocious p34cdc2 kinase activation as well; however, it does not affect the precocious accumulation of pre-MPF. Developmental competence increases in level during artificially elongated G2 phases. The results indicate that, first p34cdc2 kinase activation requires cyclin B in a complex with p34cdc2 and the presence of intact microtubuli, second establishment of competence requires a certain length of G2, and third the regulation of pre-MPF accumulation, and thus of cyclin B expression, plays a role in the relationship among growth condition, cell cycle progression, and expression of the developmental program.