Evidence for a direct role of tRNA in an amino acid transport system

The transport of phenylalanine by the general aromatic transport system in spheroplasts of Escherichia coli 9723 has been found to be stimulated by exogenous tRNA. Neither periodate-treated tRNA nor phenylalanine-charged tRNA stimulated, and the latter inhibited, phenylalanine uptake. Among preparations of specific tRNAs, tRNAPhe and tRNATyr were effective in stimulating the uptake of phenylalanine and tyrosine, respectively, and tRNAGlu and tRNAVal gave no detectable stimulation of phenylalanine or tyrosine transport. The preparation of tRNATyr was 10 times as active as unfractionated tRNA and gave as much as 167% stimulation of tyrosine transport. Correspondingly, the preparation of tRNAPhe was at least 3.5 times as active as the unfractionated tRNA and 2.5 times as active as the preparation of tRNATyr in stimulation of phenylalanine transport. Preliminary results in fractionation of the active component of tRNA for stimulating phenylalanine uptake show that the major activity resides in minor isoacceptor(s) tRNAPhe rather than the major component tRNAPhe, and the slight activity of preparations of tRNATyr is probably due to a contamination of the active tRNAPhe. Other preliminary results indicate that this type of stimulation occurs with uptake of other amino acids and their tRNA.