FREE AND CONJUGATED POLYAMINES DURING THE INITIATION OF THE CELL-CYCLE OF G0-ARRESTED MEDICAGO-VARIA CELL-CULTURES

We investigated free and conjugated polyamines after initiation of the cell cycle in Medicago varia cell cultures. Phosphate starvation caus ed an arrest of cells, preferentially in the G1 phase of the cell cycl e. During cell arrest free polyamines decreased substantially. After r efeeding of phosphate the cells retained their ability to divide. Prio r to entering S phase, free putrescine (Put) and spermidine (Spd) leve ls increased markedly. Inhibition of DNA replication could not inhibit this accumulation of free Put and Spd. On the other hand, inhibition of Put synthesis could not stop the cell cycle. The data suggest that initiation of the cell cycle from a Go state is paralleled by an incre ase in free Put and Spd levels in late G1 prior to DNA replication. In hibition of Put synthesis retarded Put accumulation but had no effect on Spd accumulation. Moreover, changes in free and conjugated Put leve ls showed different patterns during the cell cycle, pointing to distin ct physiological functions for either of the two polyamine classes. It is concluded that despite the inhibition of putrescine synthesis, the investigated cell line maintains intracellular polyamine levels that are sufficient for the progression of the cell cycle.