Construction of glycerol synthesis pathway in Klebsiella pneumoniae for bioconversion of glucose into 1,3-propanediol

Construction of Recombinant Klebsiella pneumoniae to Increase Ethanol Production on Residual Glycerol Fed-Batch Cultivations

K. pneumoniae BLh-1 strain was genetically modified aiming at obtaining high ethanol productivity in cultivations using residual glycerol from biodiesel synthesis as substrate. The recombinant strain K. pneumoniae Kp17 was obtained by inserting the multicopy plasmid pTOPOBL17 containing the AdhE gene, and its own promoter, from K. pneumoniae BLh-1. Influence of Fe²⁺ supplementation and initial glycerol concentration on culture conditions were analyzed, both in rotatory shaker and in batch bioreactors. In the bioreactor cultures, K. pneumoniae Kp17 strain produced 4.5 g L^-1 of ethanol (productivity of 0.50 g L^-1 h^-1 and yields of 0.15 g g^-1) after 24-h cultivation, corresponding to an increase of approximately 40% in ethanol concentration compared to wild strain, K. pneumoniae BLh-1. Best conditions were then applied in exponential fed-batch bioreactors, with final ethanol concentration of 17.30 g L^-1 (productivity of 0.59 g L^-1 h^-1 and yields of 0.16 g g^-1) after 30 h of feeding, representing 11.5% of increment in titer of ethanol compared to the wild strain. Mutant cells kept 92.5% of the plasmids under batch in 24 h, and 71.9% under fed-batch after 27 h of exponential feeding. The findings in this work show the possibility of using a simple approach to genetically modify K. pneumoniae to be employed this versatile bacterium for the bioconversion of residual glycerol into ethanol.

Molecular parts and genetic circuits for metabolic engineering of microorganisms

Microbial conversion of biomass into value-added biochemicals is a highly sustainable process compared to petroleum-based production. In this regard, microorganisms have been engineered via simple overexpression or deletion of metabolic genes to facilitate the production. However, the producer microorganisms require complex regulatory circuits to maximize productivity and performance. To address this issue, diverse genetic circuits have been developed that allow cells to minimize their metabolic burden, overcome metabolic imbalances and respond to a dynamically changing environment. In this review, we briefly explain the basic strategy for constructing genetic circuits by assembling molecular parts such as input, operation and output modules. Next, we describe recent applications of the circuits in the metabolic engineering of microorganisms to improve biochemical production. Beyond those achievements, genetic circuits will facilitate more innovative approaches to future strain development through mining and engineering new genetic elements and improving the complexity of genetic circuit design.

High-Level Conversion of l-lysine into Cadaverine by Escherichia coli Whole Cell Biocatalyst Expressing Hafnia alvei l-lysine Decarboxylase

Cadaverine is a C5 diamine monomer used for the production of bio-based polyamide 510. Cadaverine is produced by the decarboxylation of l-lysine using a lysine decarboxylase (LDC). In this study, we developed recombinant Escherichia coli strains for the expression of LDC from Hafnia alvei. The resulting recombinant XBHaLDC strain was used as a whole cell biocatalyst for the high-level bioconversion of l-lysine into cadaverine without the supplementation of isopropyl β-d-1-thiogalactopyranoside (IPTG) for the induction of protein expression and pyridoxal phosphate (PLP), a key cofactor for an LDC reaction. The comparison of results from enzyme characterization of E. coli and H. alvei LDC revealed that H. alvei LDC exhibited greater bioconversion ability than E. coli LDC due to higher levels of protein expression in all cellular fractions and a higher specific activity at 37 °C (1825 U/mg protein > 1003 U/mg protein). The recombinant XBHaLDC and XBEcLDC strains were constructed for the high-level production of cadaverine. Recombinant XBHaLDC produced a 1.3-fold higher titer of cadaverine (6.1 g/L) than the XBEcLDC strain (4.8 g/L) from 10 g/L of l-lysine. Furthermore, XBHaLDC, concentrated to an optical density (OD₆₀₀) of 50, efficiently produced 136 g/L of cadaverine from 200 g/L of l-lysine (97% molar yield) via an IPTG- and PLP-free whole cell bioconversion reaction. Cadaverine synthesized via a whole cell biocatalyst reaction using XBHaLDC was purified to polymer grade, and purified cadaverine was successfully used for the synthesis of polyamide 510. In conclusion, an IPTG- and PLP-free whole cell bioconversion process of l-lysine into cadaverine, using recombinant XBHaLDC, was successfully utilized for the production of bio-based polyamide 510, which has physical and thermal properties similar to polyamide 510 synthesized from chemical-grade cadaverine.