The rates of deoxyribonucleic acid (DNA) synthesis during the division cycles of the Escherichia coli strains B/r, K-12 3000, 15T−, and 15 have been measured in synchronous cultures, under several conditions of slow growth. These synchronous cultures were obtained by sucrose gradient centrifugation of exponentially growing cultures, after which the smallest cells were removed from the gradient and allowed to grow. Sucrose gradient centrifugation did not adversely affect the cell cycle, since an experiment in which an exponentially growing culture was pulsed with [3H]thymidine prior to the periodic separation and assay of the smallest cells resulted in the same conclusions, as given below. In the strains of E. coli that were studied, a decreased rate of [3H]thymidine incorporation was seen late in the cell cycle, prior to cell division. No decrease in the rate of [3H]thymidine incorporation was seen at or near the beginning of the cell cycle. Thus, all these strains appear to regulate DNA synthesis in a similar fashion during slow growth. In addition, a correlation between the appearance of cells with visible cross-walls and the start of a new round of DNA synthesis was seen, indicating that these two events might be related
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