Corynebacterium glutamicum is a natural producer of the C50 carotenoid decaprenoxanthin. The crtEcg0722crtBIYEb operon comprises most of its genes for terpenoid biosynthesis. The MarR-type regulator encoded upstream and in divergent orientation of the carotenoid biosynthesis operon has not yet been characterized. This regulator, named CrtR in this study, is encoded in many actinobacterial genomes co-occurring with terpenoid biosynthesis genes. CrtR was shown to repress the crt operon of C. glutamicum since DNA microarray experiments revealed that transcript levels of crt operon genes were increased 10 to 70-fold in its absence. Transcriptional fusions of a promoter-less gfp gene with the crt operon and crtR promoters confirmed that CrtR represses its own gene and the crt operon. Gel mobility shift assays with purified His-tagged CrtR showed that CrtR binds to a region overlapping with the −10 and −35 promoter sequences of the crt operon. Isoprenoid pyrophosphates interfered with binding of CrtR to its target DNA, a so far unknown mechanism for regulation of carotenogenesis. The molecular details of protein-ligand interactions remain to be studied. Decaprenoxanthin synthesis by C. glutamicum wild type was enhanced 10 to 30-fold upon deletion of crtR and was decreased 5 to 6-fold as result of crtR overexpression. Moreover, deletion of crtR was shown as metabolic engineering strategy to improve production of native and non-native carotenoids including lycopene, β-carotene, C.p. 450 and sarcinaxanthin.

Sarcosine, an N-methylated amino acid, shows potential as antipsychotic, and serves as building block for peptide-based drugs, and acts as detergent when acetylated. N-methylated amino acids are mainly produced chemically or by biocatalysis, with either low yields or high costs for co-factor regeneration. Corynebacterium glutamicum, which is used for the industrial production of amino acids for decades, has recently been engineered for production of N-methyl-L-alanine and sarcosine. Heterologous expression of dpkA in a C. glutamicum strain engineered for glyoxylate overproduction enabled fermentative production of sarcosine from sugars and monomethylamine. Here, mutation of an amino acyl residue in the substrate binding site of DpkA (DpkAF117L) led to an increased specific activity for reductive alkylamination of glyoxylate using monomethylamine and monoethylamine as substrates. Introduction of DpkAF117L into the production strain accelerated the production of sarcosine and a volumetric productivity of 0.16 g L−1 h−1 could be attained. Using monoethylamine as substrate, we demonstrated N-ethylglycine production with a volumetric productivity of 0.11 g L−1 h−1, which to the best of our knowledge is the first report of its fermentative production. Subsequently, the feasibility of using rice straw hydrolysate as alternative carbon source was tested and production of N-ethylglycine to a titer of 1.6 g L−1 after 60 h of fed-batch bioreactor cultivation could be attained.

In bacterial system, direct conversion of xylose to xylonic acid is mediated through NAD-dependent xylose dehydrogenase (xylB) and xylonolactonase (xylC) genes. Heterologous expression of these genes from Caulobacter crescentus into recombinant Corynebacterium glutamicum ATCC 13032 and C. glutamicum ATCC 31831 (with an innate pentose transporter, araE) resulted in an efficient bioconversion process to produce xylonic acid from xylose. Process parameters including the design of production medium was optimized using a statistical tool, Response Surface Methodology (RSM). Maximum xylonic acid of 56.32 g/L from 60 g/L xylose, i.e. about 76.67% of the maximum theoretical yield was obtained after 120 h fermentation from pure xylose with recombinant C. glutamicum ATCC 31831 containing the plasmid pVWEx1 xylB. Under the same condition, the production with recombinant C. glutamicum ATCC 13032 (with pVWEx1 xylB) was 50.66 g/L, i.e. 69% of the theoretical yield. There was no significant improvement in production with the simultaneous expression of xylB and xylC genes together indicating xylose dehydrogenase activity as one of the rate limiting factor in the bioconversion. Finally, proof of concept experiment in utilizing biomass derived pentose sugar, xylose, for xylonic acid production was also carried out and obtained 42.94 g/L xylonic acid from 60 g/L xylose. These results promise a significant value addition for the future bio refinery programs.

The non-proteinogenic amino acid 5-amino valeric acid (5-AVA) and the diamine putrescine are potential building blocks in the bio-polyamide industry. The production of 5-AVA and putrescine using engineered Corynebacterium glutamicum by the co-consumption of biomass-derived sugars is an attractive strategy and an alternative to their petrochemical synthesis. In our previous work, 5-AVA production from pure xylose by C. glutamicum was shown by heterologously expressing xylA from Xanthomonas campestris and xylB from C. glutamicum. Apart from this AVA Xyl culture, the heterologous expression of xylAXc and xylBCg was also carried out in a putrescine producing C. glutamicum to engineer a PUT Xyl strain. Even though, the pure glucose (40 g L–1) gave the maximum product yield by both the strains, the utilization of varying combinations of pure xylose and glucose by AVA Xyl and PUT Xyl in CGXII synthetic medium was initially validated. A blend of 25 g L–1 of glucose and 15 g L–1 of xylose in CGXII medium yielded 109 ± 2 mg L–1 putrescine and 874 ± 1 mg L–1 5-AVA after 72 h of fermentation. Subsequently, to demonstrate the utilization of biomass-derived sugars, the alkali (NaOH) pretreated-enzyme hydrolyzed rice straw containing a mixture of glucose (23.7 g L–1) and xylose (13.6 g L–1) was fermented by PUT Xyl and AVA Xyl to yield 91 ± 3 mg L–1 putrescine and 260 ± 2 mg L–1 5-AVA, respectively, after 72 h of fermentation. To the best of our knowledge, this is the first proof of concept report on the production of 5-AVA and putrescine using rice straw hydrolysate (RSH) as the raw material.

In bacterial system, direct conversion of xylose to xylonic acid is mediated through NAD-dependent xylose dehydrogenase (xylB) and xylonolactonase (xylC) genes. Heterologous expression of these genes from Caulobacter crescentus into recombinant Corynebacterium glutamicum ATCC 13032 and C. glutamicum ATCC 31831 (with an innate pentose transporter, araE) resulted in an efficient bioconversion process to produce xylonic acid from xylose. Process parameters including the design of production medium was optimized using a statistical tool, Response Surface Methodology (RSM). Maximum xylonic acid of 56.32 g/L from 60 g/L xylose i.e. about 76.67 % of the maximum theoretical yield was obtained after 120 h fermentation from pure xylose with recombinant C. glutamicum ATCC 31831 containing the plasmid pVWEx1 xylB. Under the same condition, the production with recombinant C. glutamicum ATCC 13032 (with pVWEx1 xylB) was 50.66g/L, i.e. 69 % of the theoretical yield. There was no significant improvement in production with the simultaneous expression of xylB and xylC genes together indicating xylose dehydrogenase activity as one of the rate limiting factor in the bioconversion. Finally, proof of concept experiment in utilizing biomass derived pentose sugar, xylose, for xylonic acid production was also carried out and obtained 42.94 g/L xylonic acid from 60 g/L xylose. These results promise a significant value addition for the future bio refinery programs.