Towards bacterial strains overproducing l-tryptophan and other aromatics by metabolic engineering

Abstract

The aromatic amino acids, L-tryptophan, L-phenylalanine, and L-tyrosine, can be manufactured by bacterial fermentation. Until recently, production efficiency of classical aromatic amino acid-producing mutants had not yet reached a high level enough to make the fermentation method the most economic. With the introduction of recombinant DNA technology, it has become possible to apply more rational approaches to strain improvement. Many recent activities in this metabolic engineering have led to several effective approaches, which include modification of terminal pathways leading to removal of bottleneck or metabolic conversion, engineering of central carbon metabolism leading to increased supply of precursors, and transport engineering leading to reduced intracellular pool of the aromatic amino acids. In this review, advances in metabolic engineering for the production of the aromatic amino acids and useful aromatic intermediates are described with particular emphasis on two representative producer organisms, Corynebacterium glutamicum and Escherichia coli.

Synthetic RNA devices to expedite the evolution of metabolite-producing microbes

“…To construct strains with different intracellular concentrations of tryptophan, we chose aroG, a 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DS) isozyme, as a target to modulate the metabolic flux towards tryptophan. Because 80% of total DS activity is known to be contributed by aroG 35 , we constructed the WT0 strain by deleting aroG in the chromosome of the W3110 strain. As expected, cellular growth rates of cells harbouring the Tryptophan Riboselector and exposed to selection pressure were severely affected by the intracellular concentrations of tryptophan (Fig.…”

“…3d). In view of the fact that the Lysine Riboselector could be utilized to select a high lysine producer, it is likely that a high tryptophan producer could be easily developed by applying the Tryptophan Riboselector to expedite evolution of tryptophan-producing E. coli 35 .…”

Analysis and Engineering of Metabolic Pathway Fluxes in Corynebacterium glutamicum

“…Due to extended research, complex and efficient [41,42]. A similar complex picture is yielded for biosynthesis of the aromatic amino acids, where enzyme inhibition by feedback regulation and negative transcriptional control are involved [6,43].…”

“…For amino acids, these supporting pathways are part of the highly interconnected network of central carbon metabolism, so that modifications will likely interfere with growth related metabolic reactions. Due to these various hurdles, classical mutagenesis and selection did not yield high levels of production, although efforts have been continuing now for almost four decades [6]. The development of recombinant DNA technologies, allowing targeted genetic modification, has initiated intensive research towards rational optimization of C. glutamicum, resulting in remarkable progress in production efficiency [30,135,136].…”

“…With the advent of recombinant DNA technology, targeted genetic engineering of C. glutamicum became available and was successfully used to derive improved production properties. Today, C. glutamicum is applied to the industrial production of glutamate as flavor enhancer in human nutrition (1.5 million tons per year), [4], lysine (850,000 tons per year) [5] as well as tryptophan (10,000 tons per year) as additive in animal feed [6]. The improvement of existing production strains focuses on higher yield and productivity, better stress tolerance, or a broader substrate spectrum.…”