When Escherichia coli was inhibited with trimethoprim in media supplemented with nucleotide bases, glycine and methionine, both RCstr and RCrel strains continued to accumulate RNA at rates very close to those in growing controls. The effects of trimethoprim on protein synthesis were studied by using as an experimental basis the rate of maturation of ribosomal particles from RNA-rich precursors. 1. In RCstr cultures given nucleotide bases but no amino acids, RNA accumulation was inhibited because of amino acid lack. However, maturation of ribosomes from their precursors was more severely inhibited than was the synthesis of rRNA. The restraints on protein synthesis were more severe at the level of translation than the transcription of operons specific for the formation of ribosomal proteins. The kinetic delay time in the passage of rRNA from RNA-rich intermediates to the final ribosome products was therefore increased some three- to four-fold. 2. In RCrel cultures in the same conditions, trimethoprim inhibition stopped ribosomal particle synthesis, but rRNA-rich precursors accumulated. 3. If glycine+methionine were also added to inhibited RCstr cultures, RNA accumulation resumed at a high rate. However, ribosomal maturation was still considerably disturbed because of a disproportionate response of the cells in the formation of protein and RNA. 4. With RCrel cultures, addition of the amino acids caused a large increase in the rate of ribosome maturation, though the degree of disproportionation between the rates of rRNA and ribosomal protein synthesis was now identical with that found in RCstr strains. 5. When inhibited RCrel cultures were supplemented, there was still a severe inhibition of protein synthesis at the level of chain initiation, but inaccuracies in the process of polypeptide chain elongation were greatly decreased. This suggests that the effects of the RCrel mutation on the fidelity of protein synthesis in bacteria are not directed at the point of chain initiation.
- © 1973 London: The Biochemical Society