Transport of aromatic amino acids and its influence on overproduction of the amino acids in Corynebacterium glutamicum

Abstract

Abstract The uptake of phenylalanine, tyrosine and tryptophan by wild-type Corynebacterium glutamicum ATCC 13032 was demonstrated to occur mainly via a common transport system by competition experiments using these amino acids which were either 14C-labeled or unlabeled. Strain HCA1 defective in the common transport system was derived from a phenylalanine auxotroph, C. glutamicum KY9182, by selecting for resistance to 5-fluorotryptophan and subsequent screening for requirement for a high concentration of phenylalanine. The wild-type gene responsible for the common transport system was cloned into a multicopy vector, based on its ability to make strain HCA1 less dependent on phenylalanine and sensitive to 5-fluorotrytophan. The resulting plasmid pCA5 conferred on strain HCA1 a fewfold increase in the uptake activities of all three aromatic amino acids relative to the wild-type levels. Although strains KY9182 and HCA1 acquired the productivity of aromatic amino acids by introducing a plasmid coexpressing a set of feedback-resistant enzymes involved in phenylalanine or trytophan biosynthesis, the production levels were higher in the HCA1 strain background than in the KY9182 background. In contrast, introduction of pCA5 into trytophan-producing C. glutamicum KY9225 resulted in a drastic decrease in the trytophan production. However, when another compatible plasmid specifying feedback-resistant tryptophan-biosynthetic enzymes coexisted in the transformant, the tryptophan productivity was restored to an appreciable level. These results indicated that prevention of amino acid transport, as well as deregulation of biosynthetic pathways, was one of the crucial factors in amino acid overproduction.