Reversible arrest of Chinese hamster V79 cells in G2 by dibutytyl AMP.

Mouse L cells 929 were cloned in supplemented Eagle's minimal medium enriched with lactalbumin and yeast extract and buffered with HEPES. Multiplication was followed photographically in single clones from the 8-cell stage through 6–7 days. Addition of the folic acid analogue methotrexate (amethopterin) in 5 × 10^?6 M concentration slowed growth only after two cell generations; 10^?4 M uridine had no effect on growth except when combined with methotrexate. The two agents together blocked cell division quickly and symptoms of thymine-less death developed in few days. The cells could be rescued before 48 h by removal of the inhibitors, or by addition of folic acid or thymidine. The combination of methotrexate with uridine blocks DNA synthesis in Tetrahymena by inhibition of thymidylate synthesis and of thymidine uptake from the complex medium. Apparently the same mechanisms operate in L cells grown in a complex medium containing thymidine.