The contest for precursors: channelling l-isoleucine synthesis in Corynebacterium glutamicum without byproduct formation

Abstract

l-Isoleucine is an essential amino acid, which is required as a pharma product and feed additive. Its synthesis shares initial steps with that of l-lysine and l-threonine, and four enzymes of l-isoleucine synthesis have an enlarged substrate specificity involved also in l-valine and l-leucine synthesis. As a consequence, constructing a strain specifically overproducing l-isoleucine without byproduct formation is a challenge. Here, we analyze for consequences of plasmid-encoded genes in Corynebacterium glutamicum MH20-22B on l-isoleucine formation, but still obtain substantial accumulation of byproducts. In a different approach, we introduce point mutations into the genome of MH20-22B to remove the feedback control of homoserine dehydrogenase, hom, and threonine dehydratase, ilvA, and we assay sets of genomic promoter mutations to increase hom and ilvA expression as well as to reduce dapA expression, the latter gene encoding the dihydrodipicolinate synthase. The promoter mutations are mirrored in the resulting differential protein levels determined by a targeted LC-MS/MS approach for the three key enzymes. The best combination of genomic mutations was found in strain K2P55, where 53 mM l-isoleucine could be obtained. Whereas in fed-batch fermentations with the plasmid-based strain, 94 mM l-isoleucine with l-lysine as byproduct was formed; with the plasmid-less strain K2P55, 109 mM l-isoleucine accumulated with no substantial byproduct formation. The specific molar yield with the latter strain was 0.188 mol l-isoleucine (mol glucose)⁻¹ which characterizes it as one of the best l-isoleucine producers available and which does not contain plasmids.