DNA turnover: Evidence from studies of steady-state bacterial populations

Since the work of Hershey (1954) most biologists have regarded DNA pools of bacterial cells as stable or conservative. Nonetheless, turnover was invoked as a possible explanation of mutation in non-replicating bacterial cells (Ryan, 1949) and in spheroplasts ( Ryan et al. , 1963 ). It also could account for observations of mutation rates independent of growth rates in slowly growing cultures of Escherichia coli ( Novick & Szilard, 1950; Fox, 1955). Results of recent studies suggest DNA turnover in cultures of primary human amnion cells ( Chang & Vetrovs, 1963), mouse fibroblasts ( Thomson et al. , 1957 ), and bacteria ( Theil & Zamenhof, 1963). We have obtained evidence of DNA turnover from studies of carbon-flux through steady-state cultures of Aerobacter aerogenes under conditions of nitrogen limitation.