TRANSPORT OF AROMATIC-AMINO-ACIDS AND ITS INFLUENCE ON OVERPRODUCTIONOF THE AMINO-ACIDS IN CORYNEBACTERIUM-GLUTAMICUM

The uptake of phenylalanine, tyrosine and tryptophan by wild-type Cory nebacterium glutamicum ATCC 13032 was demonstrated to occur mainly via a common transport system by competition experiments using these amin o acids which were either C-14-labeled or unlabeled, Strain HCA1 defec tive in the common transport system was derived from a phenylalanine a uxotroph, C. glutamicum KY9182, by selecting for resistance to 5-fluor otryptophan and subsequent screening for requirement for a high concen tration of phenylalanine, The wild-type gene responsible for the commo n transport system was cloned into a multicopy vector, based on its ab ility to make strain HCA1 less dependent on phenylalanine and sensitiv e to 5-fluorotryptophan. The resulting plasmid pCA5 conferred on strai n HCA1 a fewfold increase in the uptake activities of all three aromat ic amino acids relative to the wild-type levels, Although strains KY91 82 and HCA1 acquired the productivity of aromatic amino acids by intro ducing a plasmid coexpressing a set of feedback-resistant enzymes invo lved in phenylalanine or tryptophan biosynthesis, the production level s were higher in the HCA1 strain background than in the KY9182 backgro und, In contrast, introduction of pCA5 into tryptophan-producing C. gl utamicum KY9225 resulted in a drastic decrease in the tryptophan produ ction. However, when another compatible plasmid specifying feedback-re sistant tryptophan-biosynthetic enzymes coexisted in the transformant, the tryptophan productivity was restored to an appreciable level, The se results indicated that prevention of amino acid transport, as well as deregulation of biosynthetic pathways, was one of the crucial facto rs in amino acid overproduction.