MIMOSINE ARRESTS THE CELL-CYCLE AFTER CELLS ENTER S-PHASE

L-Mimosine beta-N-[3-hydroxy-4-pyridone]-alpha-aminopropionic acid)-a rare amino acid derived from Mimosa and Leucaena plants-arrests cells reversibly late during G1 phase or at the beginning of S-phase. If mim osine were to arrest cells immediately before S-phase, it would provid e a superb tool for the investigation of the initiation of DNA synthes is, Therefore, we reexamined the point of action of mimosine. Mitotic HeLa cells were released into 200 mu M mimosine and grown for similar to 10 h to block them, before the cells were permeabilized and the ami no acid removed by washing them thoroughly. On addition of the appropr iate triphosphates, DNA synthesis-measured by the incorporation of [P- 32]dTTP-began immediately; as it is known that such permeabilized cell s cannot initiate DNA synthesis but can only resume elongating previou sly initiated chains, mimosine must arrest after DNA synthesis has beg un. Moreover, cells grown in mimosine assembled functional replication factories-detected by immunolabeling after incorporation of biotin-dU TP-that were typical of those found early during S-phase. Disappointin gly, it seems that mimosine-like aphidocolin-blocks only after cells e nter S-phase. (C) 1996 Academic Press, Inc.