Suspension-cultured mouse plasmacytoma cells (MPC-11) were accumulated in the late G1 phase by exposure to isoleucine-deficient medium for 20-24 h. The arrested culture was fed with complete medium enabling the cells to continue the cell cycle synchronously, undergo mitosis, and enter a second cycle of growth. This method of synchronization left the protein-synthesizing ability intact as judged by the polysome profile and the capacity of the cells to incorporate labeled amino acids into protein after the restoration of isoleucine. After incubation in isoleucine-deficient medium and the addition of isoleucine to the culture, cells entered the S phase after a short lag, as judged by [3H]thymidine incorporation into nucleic acid and by spectrophotometric measurement of nuclear DNA. The cells were in mitosis between 12 and 18 h as judged by the increase in cell count and analysis of cell populations on albumin gradients. Synthesis and secretion of light-chain immunoglobulin were maximal in the late G1/early S phase of the first cycle. During late S phase, G2 phase, and mitosis, both synthesis and secretion were observed to be at a low level; however, immediately after motosis the cells which then entered the G1 phase apparently commenced synthesis of light chain immunoglobulin straight away, although secretion of labeled material remained at a low level.

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