Although Dictyostelium differentiation occurs in the absence of external nutrients, two periods of mitosis occur, one during early development and one during the formation of the migrating pseudoplasmodium. We showed previously that
cyclin B mRNA levels vary in a cell cycle dependent manner during vegetative cell growth. In the present study, we report that
cyclin B mRNA levels change dramatically during development, reaching a maximum at the tipped aggregate stage. However, amounts of
cyclin B protein vary only slightly, peaking during early development and decreasing during late aggregation and pseudoplasmodial formation. Cdc2
protein levels also remain relatively constant during development. Cdc2-histone H1
kinase activity was considerably higher in vegetative
cell extracts of transformants that expressed large amounts of truncated
cyclin B protein in comparison to extracts of the parental Ax-2 cells. These results suggest that Cdc2
kinase activity is dependent upon the level of
cyclin B in vegetative cells. This result is consistent with the idea that variations in the level of
cyclin B during growth regulate the cell cycle. When Cdc2
histone H1 kinase activity was determined during development, it was also found that activity correlated reasonably well with the amount of
cyclin B protein. Thus, there was an increase in Cdc2
histone H1 kinase activity early in development, and then levels decreased as development progressed. The increase in Cdc2
histone H1 kinase activity that occurs early in development following
starvation may be important in accelerating G2-phase cells through into mitosis. There was no increase in Cdc2
histone H1 kinase that accompanied the previously reported late developmental mitosis.(ABSTRACT TRUNCATED AT 250 WORDS)