Changes in the cell content and rate of synthesis of mRNA were studied in auxotrophs of Escherichia coli recovering from a period of amino acid deprivation. Parallel studies were carried out on bacterial strains inhibited with trimethoprim, when glycine and methionine were added to relieve an amino acid deficiency. In the latter case, protein synthesis was still severely inhibited through a lack of N-formylmethionyl-tRNAfMet for chain initiation, so that fewer ribosomes were attached to mRNA chains. (1) In RCstr strains recovering from amino acid starvation, there was a transient oversynthesis of mRNA, but the amounts returned to normal after about a 15-min period of recovery. RCrel strains did not show this effect; any extra mRNA accumulated during the previous starvation period was rapidly lost, but no oversynthesis occurred during the resumption of growth. (2) In trimethoprim-inhibited cultures supplemented with glycine and methionine, mRNA was produced at the same rate, relative to stable RNA species, as during normal growth. The evidence implied that decreased rates of ribosome attachment had no effect on the functional or chemical lifetime of the mRNA fraction. This suggests that mRNA stability does not depend on the frequency of translation by ribosomes. (3) Changes in the mRNA contents of trimethoprim-inhibited RCstr and RCrel cultures were noted soon after supplementation with glycine and methionine. These closely followed those observed in cultures recovering from simple amino acid withdrawal.
