Differential utilization of leucyl-tRNAs by Escherichia coli.

The utilization of the isoaccepting species of leucyl-tRNA in protein synthesis has been examined in E. coli. Two minor leucyl-tRNA species, isoacceptors tRNA3Leu and tRNA4Leu, are the predominant species found bound to ribosomes during exponential growth in minimal medium. In rich medium, an increased proportion of tRNA1Leu is found bound to ribosomes. One species, tRNA5Leu, is always absent from ribosomes. In a mutant strain in which normal tRNA3Leu and tRNA5Leu are reduced or absent, tRNA1Leu is a major ribosome-bound species in minimal medium. Protein synthesis in vitro with RNA from E. coli further suggests that tRNA5Leu is rarely used for total protein synthesis; however, this species is active with RNA from MS2 phage. We propose that tRNA1Leu can substitute for tRNA3Leu under rapid growth conditions, and that tRNA5Leu is used minimally in total protein synthesis.