SUMMARY: The exponential growth rate of a streptomycin-dependent strain of was proportional to streptomycin concentration until a critical concentration was reached, above which it was independent of streptomycin concentration. The value of the critical concentration changed with a change either in the carbon source, or in the temperature of cultivation. Below the critical concentration, the macromolecular composition of the cells was affected by the external streptomycin concentration: as this decreased, the ribonucleic acid (RNA) content of the organisms increased, and the protein content decreased. When external streptomycin was removed, streptomycin-dependent organisms continued to grow for many hours. Growth was at first exponential, the extent and duration of this phase being functions of the concentration of streptomycin to which the organisms had previously been exposed. This phase was followed by a much longer period of arithmetic growth, unaccompanied by cell division, during which the organisms elongated progressively. Growth in the absence of streptomycin caused changes in the macromolecular composition of the organisms which were similar in nature to those produced by growth with a subcritical concentration of streptomycin, but much more pronounced. The greatly increased total RNA content of these organisms was not accompanied by grossly detectable qualitative changes in the RNA content of the organisms. In the absence of streptomycin, the synthesis of some enzymes was either arrested or decreased in rate; the synthesis of others was unaffected. This leads to an imbalance in the enzymic constitution. These differential effects on enzyme synthesis appeared to be random. Growth in absence of streptomycin did not seem to affect deoxyribonucleic acid (DNA) synthesis or function, as shown by the ability of a lysogenic streptomycin-dependent strain to produce infective phage under such conditions. The re-introduction of streptomycin to a culture growing arithmetically as a consequence of streptomycin depletion caused a resumption of DNA synthesis at the normal exponential rate. The rate of protein synthesis also soon increased, but attained its normal exponential rate more slowly. RNA synthesis was wholly arrested until the RNA content of the organisms had fallen to a normal value, and then resumed at the normal exponential rate. Grown in the presence of a greater than critical concentration of streptomycin, the streptomycin-dependent organism bound irreversibly about 250,000 molecules of streptomycin, half of which could be extracted with hot water, and the remainder with hot perchloric acid. A new hypothesis concerning the location and nature of the genetically determined intracellular lesion which results in streptomycin dependence is developed on the basis of these facts.


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