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Role of tRNALeu in Branched-chain Amino Acid Transport

Steven C. Quay, Dale L. Oxender


The intracellular levels of most nutrients are carefully controlled to meet the varying demands for these during the normal growth of the cell. To control the intracellular level of an amino acid, the cell must balance the processes that lead to increases in the nutrient concentration, such as transport and biosynthesis, with those that tend to decrease it, such as metabolism and macromolecular synthesis.

The regulation of the levels of the branched-chain amino acids, leucine, isoleucine, and valine, in bacteria and mammalian cells has been the subject of numerous studies (for reviews, see Umbarger 1973; Quay and Oxender 1979, 1980). These studies indicate a complex system for regulation of biosynthesis and transport involving amino acids, tRNA molecules and their synthetases, transcriptional termination factors, and the products of structural genes for regulatory loci. This review discusses the experimental evidence that leads to the hypothesis that tRNALeu is involved in branched-chain amino acid transport in both bacteria and mammalian cells.

Aminoacyl-tRNA synthetases and their cognate tRNA species have been implicated in the regulation of amino acid biosynthetic enzymes for histidine (Brenner and Ames 1971), leucine (Low et al. 1971), isoleucine and valine (Umbarger 1973), arginine (Williams 1973), tryptophan (Bertrand et al. 1975), and methionine (Brenchley and Williams 1975). In a comprehensive study, the regulation of transport was tested in strains with mutations in the valyl-, isoleucyl-, and leucyl-tRNA synthetases to test whether these proteins were effectors in this regulation mechanism.

Strain NP29 valSts exhibits a temperature-sensitive phenotype that...

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