Acanthamoeba castellanii cells become growth arrested at different positions of the G2 phase of the cell cycle. Cells arresting at the competence position of G2 either develop into cysts in response to starvation or enter the mitotic division cycle in response to fresh nutrient medium. In cells arresting elsewhere in G2 growth can be initiated. The cell cycle position at which cells become growth arrested is dependent on the mode of cell cycle progression which, in turn, is strongly dependent on physical parameters of the cultivation conditions. Cells arresting at the competence position progress through cell cycles in which developmental competence and the so-called ‘growth-associated’ or ‘M phase-specific’ histone H1 kinase activity oscillate in levels. H1 kinase activity increases rapidly at the onset of mitosis and decreases to its basal level immediately after the completion of mitosis. In this case, peak levels of developmental competence slightly precede peak levels of H1 kinase activity. In contrast, cells arresting elsewhere in G2 are advanced through their cell cycle, i.e. undergo mitosis at reduced G2 phase length and smaller cell size compared with normal. The premature mitosis correlates in time with a rapid increase of H1 kinase activity. However, the extent of its activation is increased and its Lnactivation is slower so that H1 kinase activity does not decrease to its basal level during the shortened G2 phase. In this case a cell cycle period in which establishment of developmental competence takes place is lacking. In response to cycloheximide or vanadate, which delay the onset of premature mitosis, the competence period can be restored. H1 kinase activity is likely to represent p34cdc2 protein kinase activity, which has been shown in a wide variety of eukaryotic cell types to play a key role in regulating mitosis. Therefore, the results indicate that the mechanisms regulating development are coupled to controls acting over mitosis. In vitro translation patterns suggest that the length of G2 and/or a low basal level of H1 kinase activity is (are) important for regulating the cellular content of competence- and development-specific RNA species.

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