Hyperproduction of L-threonine by an Escherichia coli mutant with impaired L-threonine uptake.

An efficient production strain for L-threonine fermentation was derived from Escherichia coli by multiple rounds of mutation programs that aimed at deregulation of the L-threonine biosynthetic pathway and blocking of L-threonine degradation pathways. When the optimum amount of DL-methionine was added, this strain KY10935, an L-methionine auxotroph, gave 100 g/liter L-threonine after 77 h cultivation. In this strain, key enzymes in the L-threonine biosynthetic pathway were highly derepressed, but some were inhibited by lower concentrations of L-threonine than the accumulated level. Such incomplete deregulation of the pathway was accounted for by the intracellular concentration of L-threonine being lower than the extracellular level. In an assessment of L-threonine transport in terms of phenotypic growth responses to the amino acid, L-threonine-auxotrophic mutants with a lesion in the L-threonine operon were derived from strain KY10935 by selection for auxotrophy for dipeptide L-alanyl-L-threonine or glycyl-L-threonine, the transport systems of which were different from those of L-threonine. All three independent mutants isolated needed an extraordinarily high concentration (10 mg/ml) of L-threonine, but grew in the presence of a low concentration (10 micrograms/ml) of either dipeptide, indicating that strain KY10935 had impaired L-threonine uptake. These results suggested that the strain had an unusual mechanism of L-threonine hyperproduction: the inability to take up L-threonine that had accumulated extracellularly decreased the steady-state level of intracellular L-threonine, freeing the remaining regulatory steps of feedback inhibition.