Mimosine Arrests the Cell Cycle after Cells Enter S-Phase

-Mimosine (β-N-[3-hydroxy-4-pyridone]-α-aminopropionic acid)—a rare amino acid derived fromMimosaandLeucaenaplants—arrests cells reversibly late during G1 phase or at the beginning of S-phase. If mimosine were to arrest cells immediately before S-phase, it would provide a superb tool for the investigation of the initiation of DNA synthesis. Therefore, we reexamined the point of action of mimosine. Mitotic HeLa cells were released into 200 μMmimosine and grown for 10 h to block them, before the cells were permeabilized and the amino acid removed by washing them thoroughly. On addition of the appropriate triphosphates, DNA synthesis—measured by the incorporation of [³²P]dTTP—began immediately; as it is known that such permeabilized cells cannot initiate DNA synthesis but can only resume elongating previously initiated chains, mimosine must arrest after DNA synthesis has begun. Moreover, cells grown in mimosine assembled functional replication factories—detected by immunolabeling after incorporation of biotin–dUTP—that were typical of those found early during S-phase. Disappointingly, it seems that mimosine—like aphidocolin—blocks only after cells enter S-phase.