Seven UV-sensitive clones were isolated from the mouse cell lines, FM3A and L5178Y, using Lederberg style replica plating. The UV-sensitive clones were found at a frequency of 1 in 886 clones of FM3A and of 6 in 420 clones of L5178Y, after treatment with N-methyl-N'-nitro-N-nitrosoguanidine, but no UV-sensitive clones were obtained in the untreated controls. The Do values of freshly isolated clones were 1 0 to 2 1 ergs/mmZ (mainly 10 to 13 ergs/ mmz), while the original FM3A and L5178Y lines had Do values of 32 ergs/mm2. The UV-sensitive clones were found to be rather stable for a period ranging from two to eight months after isolation. However, later some of them tended to lose their UV-sensitivity, in terms of increase in Do values or n values. More stable UV-sensitive subclones could be isolated from these populations by cloning using the replica plating method. Caffeine was found to potentiate the lethal action of UV-irradiation on the UV-sensitive clones as well as on the original FM3A cells.

ABSTRACT. Fibroblast-like cells from a BALB/c mouse embryo were propagated and transferred weekly in culture with a constant inoculation density of 5 x 103 cells/cm2. In this culture schedule a period of decreased proliferation (crisis) continued for 3 to 4 weeks before the initiation of continuous cell proliferation. This provided an experimental system suitable for studying proliferation conversion. The number and the proliferation rate of clonogenic cells increased early in crisis. Production of the colony-stimulating factor for mouse bone marrow cells by fibroblast-like cells was enhanced during the period of decreasing proliferation and crisis, then it decreased when continuous proliferation began. The results indicate that cells with infinite proliferation potential appear early in crisis, although the major population of cells terminates proliferation during crisis and enhances differentiated functions.The initial phase of proliferation of mammalian fibroblast-like cells in culture is not directly followed by continuous proliferation (7,17). For human and chicken fibroblasts, initial cell proliferation rarely is followed by continuous proliferation (3,4,12). For rodent cells, the initial proliferation often is followed by a period of virtually no increase in cell population (crisis), after which there is continuous proliferation (6,18,19,21). The proliferation conversion described above has been regarded as a feature of in vitro transformation (17).During proliferation conversion, changes in chromosomal constitution, the rate of DNA synthesis, cell volume and cell morphology have been observed (1,6,13,18,19,21), but functional changes associated with this conversion rarely have been found. Difficulties in studying conversion have been to establish a cell culture system which permits kinetic analyses, and to determine the functional parameters during conversion.A mouse fibroblast culture system is described, in which the phase of initial cell proliferation and the subsequent phase of continuous proliferation are separated sufficiently by a period of "crisis". During conversion, we could measure the production of the colony stimulating factor (CSF) for mouse bone marrow cells, because CSF production is a function expressed by cultured mouse fibroblasts (16,20). Parameters such as the rate of cell proliferation, and the proportion of clonogenic cells in the cell population and their relationship to CSF production are described.