RELATIONSHIP BETWEEN PROTEIN-SYNTHESIS AND CONCENTRATIONS OF CHARGED AND UNCHARGED TRANSFER RNATRP IN ESCHERICHIA-COLI

We have continuously monitored Trp-tRNATrp concentrations in vivo and, in the same cultures, measured rates of protein synthesis in isogenic stringent and relaxed strains. We have also manipulated cellular charged and uncharged [tRNATrp] by two means: (i) the strain used contains a Trp-tRNA synthetase mutation that increases the Km for Trp; thus, varying exogenous Trp varies cellular Trp-tRNATrp; and (ii) we have introduced into the mutant strain a plasmid containing the tRNATrp gene behind an inducible promoter; thus, total [tRNATrp] also can be varied depending on length of induction. The use of these conditions, combined with a previously characterized assay system, has allowed us to demonstrate that (i) the rate of incorporation of Trp into protein is proportional to the fraction of tRNATrp that is charged; for any given total [tRNATrp], this rate is also proportional to the [Trp-tRNATrp]; (H) uncharged tRNATrp inhibits incorporation of Trp into protein; and (iii) rates of incorporation into protein of at least two other amino acids, Lys and Cys, are also sensitive to [Trp-tRNATrp] and are inhibited by uncharged tRNATrp. Our results are consistent with models of translational control that postulate modulating polypeptide chain elongation efficiency by varying concentrations of specific tRNAs.