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We have recently described the synthesis of nucleic acid and of the polyamines putrescine and spermidinel" 2 in a stringent derivative of Escherichia coli strain 15 requiring thymine, arginine, and uracil (TAU st).3 4 The elimination of arginine from the medium not only results in the inhibition of RNA synthesis by about 90 per cent, thereby defining "stringency," but also prevents the cellular accumulation of spermidine.' The relaxation of ribosomal RNA synthesis in TAU st is effected by streptomycill2 which concomitantly kills the cells, expels cellular putrescine, and causes an increase of cellular spermidine. l We have suggested that the lethality of streptomycin requires the synthesis of ribosomal RNA.5Exogenous spermidine itself relaxes RNA synthesis extensively in cultures of TAU lacking arginine;' RNA accumulated in the presence of spermidine is predominantly ribosomal RNA (unpublished data). It appears then that the synthesis of ribosomal RNA, but not of DNA,2 in stringent E. coli is correlated with both the absolute level of cellular spermidine and the relative level of spermidine to total polyamine. Such an effect on RNA synthesis in vivo appears at least superficially similar to the initiation of RNA synthesis in vitro by addition of spermidine to a stalled RNA polymerase, as summarized elsewhere.2 As discussed earlier,2 an isogenic pair of relaxed and stringent auxotrophic strains might provide a useful system for the study of the relation of polyamines to RNA synthesis. A relaxed derivative of E. coli strain TAU st has recently become available. Our comparative analyses of the relaxed (TAU rel) and the stringent (TAU st) organisms have now revealed that TAU rel makes far more spermidine than does TAU st. Furthermore, spermidine synthesis in TAU rel is independent of RNA accumulation. During amino acid starvation, an accumulation of intracellular spermidine in TAU rel sharply inhibits putrescine biosynthesis. It appears also that relaxed strains are hypersensitive to streptomycin in amino acid deficiency, such sensitivity being apparently dependent on the production of ribosomal RNA. We also have observed that the addition of methionine, the spermidine precursor, to TAU st also increases the effect of streptomycin in both stimulating RNA synthesis and the lethality of the antibiotic.Materials and Methods.-Spermidine trihydrochloride, uracil-2-C14, and thymine-2-Cl4 were purchased from the California Corporation for Biochemical Research. The spermidine trihydrochloride is recorded as having a melting point of 261-2620, N = 16.30 per cent; we have found by chromatography and paper electrophoresis that this material contains less than 1 per centA of any ninhydrinreactive impurity. Putrescine-1,4-Cl4 -2 HCl was obtained from New England Nuclear Corporation. Streptomycin sulfate was obtained from the Nutritional Biochemicals Corporation. 721

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