Subcompartments of the G1 phase of cell cycle detected by flow cytometry

Cellular DNA and RNA were simultaneously quantitated in individual cells of cycling and quiescent populations by flow cytometry. Based on differences in RNA content, two distinct subcompartments, G1A and G1B, were recognized in the G1 phase of exponentially growing cells. After mitosis, cells reside in the low-RNA, G1A compartment from which they cannot enter the S phase directly. An increase in RNA above a critical level is required for G1 cells to be able to initiate DNA replication; G1 cells with RNA values above this level are classified as G1B. Cell transition from G1A to G1B and from G1B to S was analyzed in a stathmokinetic experiment on L1210 cells with a doubling time of 10 hr. The half-time of cell residence in the indeterminate state of G1 was found to be 1 hr, and the duration of the deterministic portion of G1 phase was 2 hr. The indeterminate state, although not identical with G1A, is most likely located in G1A. Cell quiescence induced by serum deprivation (3T3 cells) or by addition of n-butyrate (L1210) results in cell arrest at a state which, judged by RNA content, is similar to that of G1A of exponentially growing cells. The exit from this state, however, is much slower after stimulation of these blocked cells than the transition of G1A to G1B in cells growing exponentially. Thymidine or hydroxyurea arrest cells in G1B or perhaps at the border between G1B and S. Prolonged incubation of 3T3 cells to confluence results in a marked loss of cellular RNA below the level of the G1A state. This deep quiescent state (G1Q) is distinctly different from the G1A state of cycling cells or cells blocked in G1A by serum deprivation.