The role of nuclear proteins in RNA synthesis

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When epithelial cells from baby mice are cultured on a solid substrate in vitro, DNA synthesis and cell division take place in a majority of the cells 24 to 48 h after inoculation. This cell proliferation is regularly preceded by a sequence of nuclear changes likely to be involved in gene activation during the process of growth-transformation. These nuclear changes, which in the present investigation were studied by quantitative cytochemical methods, are characterized by: (1) increased binding of acridine orange (AO) very soon after cell attachment, reflecting initial changes of the deoxyribonucleoprotein (DNP) complex; (2) accumulation of proteins in the cell nucleus; (3) dispersion of the nuclear chromatin; (4) increased rate of 14C-uridine incorporation. The initial DNP change, reflected by the increased AO-binding, was per se not a sufficient factor for stimulation of 14C-uridine incorporation. An increased 3H-uridine incorporation was only observed when protein began to accumulate in the nucleus and when the chromatin changed from a condensed to a more dispersed state. The rate of 14C-uridine incorporation was, during the whole process of growth transformation, found to be directly proportional to the amount of protein that had accumulated in the nucleus. However, the initial DNP change was a necessary prerequisite for the subsequent accumulation of protein in the nucleus. Attachment of the cells to the solid substrate was found to be necessary in expressing this sequence of nuclear events. Furthermore, in areas on the glass slide where many cells had attached, these nuclear changes occurred more rapidly.