Polyamines and the Accumulation of Ribonucleic Acid in Some Polyauxotrophic Strains of Escherichia coli

Abstract

The cellular accumulations of the polyamlnes and RNA were compared in the polyauxotrophic mutants of E. coli strain 15 TAU and E. coli K-12 RCrel met- leu". Putrescine, spermidine and their monoacetyl derivatives were the main polyamines in both strains, when grown in glucose-mineral medium. No significant degradation of either 14C-putrescine or 14C -spermidine was found in growing cultures of strain 15 TAU, which requires thymine, arginine and uracil for growth. Experiments with this organism showed that in a variety of different incubation conditions, which included normal growth, amino acid starvation, inhibition by chloramphenicol or streptomycin, or thymine deprivation, a close correlation was seen between the intracellular accumulation of uncon-jugated spermidine and RNA. In the presence of arginine, the antibiotics stimulated the production of putrescine and spermidine/unit of bacterial mass. Deprivation of arginine also resulted in an increase in the production of putrescine/unit of bacterial mass, most of which was excreted into the growth medium. However, in this system the antibiotics reduced the synthesis of putrescine. Furthermore, streptomycin caused a rapid loss of cellular putrescine into the medium. The latter effect was not seen in anaerobic conditions or in a streptomycin-resistant mutant of 15 TAU. Methionine added to the growth medium of growing TAU not only markedly increased the total production of spermidine, but also increased both the intracellular concentration of spermidine and the accumulation of RNA. Exogenous spermidine extensively relaxed RNA synthesis in amino acid-starved cultures of 15 TAU. Analysis in sucrose density gradients showed that the RNA accumulated in the presence of spermidine was ribosomal RNA. Cells of E. coli K-12 RCrel met" leu", grown in a complete medium, had approximately the same ratio of free spermidine to RNA as did strain 15 TAU. However, the relaxed strain showed a much lower ratio of putrescine to spermidine than the stringent 15 TAU. Omission of methionine stopped spermidine synthesis and markedly increased both the intracellular accumulation and the total production of putrescine. It seems that a high intracellular level of spermidine acts as a feedback inhibitor in the biosynthesis of putrescine in this strain. The hypothesis that the intracellular concentration of polyamines may participate in the control of the synthesis of ribosomal UNA in bacteria is discussed.