Construction of T7-Like Expression System in Pseudomonas putida KT2440 to Enhance the Heterologous Expression Level

XylR Overexpression in Escherichia coli Alleviated Transcriptional Repression by Arabinose and Enhanced Xylitol Bioproduction from Xylose Mother Liquor

Xylitol is a polyol widely used in food, pharmaceuticals, and light industries. It is currently produced through the chemical catalytic hydrogenation of xylose and generates xylose mother liquor as a substantial byproduct in the procedure of xylose extraction. If xylose mother liquor could also be efficiently bioconverted to xylitol, the greenness and atom economy of xylitol production would be largely improved. However, xylose mother liquor contains a mixture of glucose, xylose, and arabinose, raising the issue of carbon catabolic repression in its utilization by microbial conversion. Targeting this challenge, the transcriptional activator XylR was overexpressed in a previously constructed xylitol-producing E. coli strain CPH. The resulting strain CPHR produced 16.61 g/L of xylitol in shake-flask cultures from the mixture of corn cob hydrolysate and xylose mother liquor (1:1, v/v) with a xylose conversion rate of 90.1%, which were 2.23 and 2.15 times higher than the starting strain, respectively. Furthermore, XylR overexpression upregulated the expression levels of xylE, xylF, xylG, and xylH genes by 2.08-2.72 times in arabinose-containing medium, suggesting the alleviation of transcriptional repression of xylose transport genes by arabinose. This work lays the foundation for xylitol bioproduction from xylose mother liquor.

Development of efficient modules for recombinant protein expression and periplasmic localiszation in Pseudomonas bharatica CSV86T

Escherichia coli has been widely employed as a host for heterologous protein expression. However, due to certain limitations, alternative hosts like Pseudomonas, Lactococcus and Bacillus are being explored. Pseudomonas bharatica CSV86^T, a novel soil isolate, preferentially degrades wide range of aromatics over simple carbon sources like glucose and glycerol. Strain also possesses advantageous eco-physiological traits, making it an ideal host for engineering xenobiotic degradation pathways, which necessitates the development of heterologous expression systems. Based on the efficient growth, short lag-phase and rapid metabolism of naphthalene, Pnah and Psal promoters (regulated by NahR) were selected for expression. Pnah was found to be strong and leaky as compared to Psal, using 1-naphthol 2-hydroxylase (1NH, ∼66 kDa) as reporter gene in strain CSV86^T. The Carbaryl hydrolase (CH, ∼72kDa) from Pseudomonas sp. C5pp was expressed under Pnah in strain CSV86^T and could successfully be translocated to the periplasm due to the presence of the Tmd + Sp sequence. The recombinant CH was purified from the periplasmic fraction and the kinetic characteristics were found to be similar to the native protein from strain C5pp. These results potentiate the suitability of P. bharatica CSV86^T as a desirable host, while Pnah and the Tmd + Sp can be employed for overexpression and periplasmic localisation, respectively. Such tools find application in heterologous protein expression and metabolic engineering applications.

Assessing the Orthogonality of Phage-Encoded RNA Polymerases for Tailored Synthetic Biology Applications in Pseudomonas Species

The phage T7 RNA polymerase (RNAP) and lysozyme form the basis of the widely used pET expression system for recombinant expression in the biotechnology field and as a tool in microbial synthetic biology. Attempts to transfer this genetic circuitry from Escherichia coli to non-model bacterial organisms with high potential have been restricted by the cytotoxicity of the T7 RNAP in the receiving hosts. We here explore the diversity of T7-like RNAPs mined directly from Pseudomonas phages for implementation in Pseudomonas species, thus relying on the co-evolution and natural adaptation of the system towards its host. By screening and characterizing different viral transcription machinery using a vector-based system in P. putida., we identified a set of four non-toxic phage RNAPs from phages phi15, PPPL-1, Pf-10, and 67PfluR64PP, showing a broad activity range and orthogonality to each other and the T7 RNAP. In addition, we confirmed the transcription start sites of their predicted promoters and improved the stringency of the phage RNAP expression systems by introducing and optimizing phage lysozymes for RNAP inhibition. This set of viral RNAPs expands the adaption of T7-inspired circuitry towards Pseudomonas species and highlights the potential of mining tailored genetic parts and tools from phages for their non-model host.

Targeting Transcriptional and Translational Hindrances in a Modular T7RNAP Expression System in Engineered Pseudomonas putida

The T7 RNA polymerase is considered one of the most popular tools for heterologous gene expression in the gold standard biotechnological host Escherichia coli. However, the exploitation of this tool in other prospective hosts, such as the biotechnologically relevant bacterium Pseudomonas putida, is still very scarce. The majority of the existing T7-based systems in P. putida show low expression strengths and possess only weak controllability. A fundamental understanding of these systems is necessary in order to design robust and predictable biotechnological processes. To fill this gap, we established and characterized a modular T7 RNA polymerase-based system for heterologous protein production in P. putida, using the enhanced Green Fluorescent Protein (eGFP) as an easy-to-quantify reporter protein. We have effectively targeted the limitations associated with the initial genetic setup of the system, such as slow growth and low protein production rates. By replacing the T7 phage-inherent TΦ terminator downstream of the heterologous gene with the synthetic tZ terminator, growth and protein production rates improved drastically, and the T7 RNA polymerase system reached a productivity level comparable to that of an intrinsic RNA polymerase-based system. Furthermore, we were able to show that the system was saturated with T7 RNA polymerase by applying a T7 RNA polymerase ribosome binding site library to tune heterologous protein production. This saturation indicates an essential role for the ribosome binding sites of the T7 RNA polymerase since, in an oversaturated system, cellular resources are lost to the synthesis of unnecessary T7 RNA polymerase. Eventually, we combined the experimental data into a model that can predict the eGFP production rate with respect to the relative strength of the ribosome binding sites upstream of the T7 gene.

Plasmids for Controlled and Tunable High-Level Expression in E. coli

Genetic systems for protein overexpression are required tools in microbiological and biochemical research. Ideally, these systems include standardized genetic parts with predictable behavior, enabling the construction of stable expression systems in the host organism.