Streptomycin Dependence in Escherichia coli: Effects of Antibiotic Deprivation on Ribosomes

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RESUMO

The inhibition of cell division and the ultimate loss of viability after removal of streptomycin from growing cultures of streptomycin-dependent bacteria are not the result of “unbalanced growth” or of the breakdown of ribosomes. The streptomycin-dependent strain of Escherichia coli K-12 studied continued to synthesize ribonucleic acid (RNA) and protein during streptomycin starvation. There was no evidence of a gross imbalance in the ratio of RNA to protein synthesized or of selective degradation of either protein or RNA. Using the sedimentation of subunits in sucrose as the criterion, normal ribosomes were synthesized even after 18 h of streptomycin deprivation, although the rates of appearance of mature 30S and 50S subunits decreased with time of deprivation. Once formed, these ribosomes appeared stable, as did those synthesized before the onset of starvation. Ribosomes isolated from starved dependent cells were as “functional” as ribosomes from cells grown with streptomycin in their capacity to bind aminoacyl-transfer RNA in response to polyuridylic acid or natural messenger RNA to interconvert between active and inactive transfer RNA binding states, and to synthesize proteins in cell-free systems. The effects are consistent with an impaired rate of synthesis of ribosomal components or assembly of ribosomes resulting in a continually diminishing rate of protein synthesis. The effect on cell division may be the result of a decreased rate of protein synthesis in general and the requirement for a specific protein(s) in particular.

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