RNA synthesis in synchronously growing populations of HeLa S3 cells. I. Rate of total RNA synthesis and its relationship to DNA synthesis

The rate of RNA synthesis in synchronously growing HeLa S3 cells was determined as a function of position in the cell generation cycle. Measurements throughout the cycle of both the rate of incorporation of radioactively‐labeled uridine and of the total amount of RNA indicate that (1) the rate of RNA synthesis is constant (or increases only slightly) during G 1 , approximately doubles during the first half of S, and then remains constant during the remainder of S and G 2 , and (2) cells attain the average G 1 rate of RNA synthesis very early in G 1 , and maintain the average G 2 rate until mitosis. If the initiation of DNA synthesis is blocked, the acceleration of RNA synthesis is markedly reduced or eliminated. Further experiments in which DNA synthesis was inhibited at different times in S, or to varying degrees from the beginning of S, suggest that the extent to which RNA synthesis is accelerated depends on the amount of DNA duplicated. These data also indicate that duplication of the first half, and in particular the first few per cent, of the DNA complement results in a disproportionate acceleration of RNA synthesis. The possibility that fluctuations in the sizes of precursor pools may lead to misinterpretation of labeled‐uridine incorporation data was examined. Experiments indicate that in this system pool fluctuations do not cause invalid measures of RNA synthesis. It is concluded that RNA synthesis occurs throughout interphase, but undergoes a two‐fold increase in rate which is dependent on the duplication of DNA.