Construction of a Robust Sphingomonas sp. Strain for Welan Gum Production via the Expression of Global Transcriptional Regulator IrrE.

Xiaoliu Liu,Hong Xu,Ming Zhao,Zheng Xu,Sha Li

doi:10.3389/fbioe.2020.00674

Abstract

Welan gum is a widely used microbial polysaccharide produced by Sphingomonas sp. However, an important factor hindering the expansion of its production is the maladaptation of strain to fermentation conditions. In this work, the global transcriptional regulator gene irre was selected as a stress-resistant element. And it was integrated into the site of the genomic carotene synthesis key enzyme gene crtB to construct a robust carotenoid-free welan gum producing strain. Fermentation with the recombinant strain effectively reduced the ethanol consumption and pigment content in the product. The tolerance temperature increased by 10°C without the need for controlling the pH. Under this fermentation condition, welan gum concentration could still reach 20.26 ± 0.25 g/L, which was 187.38% higher than that of the wild-type strain (7.05 ± 0.15 g/L). Transcriptome analysis showed that with the control of IrrE, more than 1000 genes that are involved in multiple pathways, including two-component system, bacterial chemotaxis, flagellar assembly, and cell cycle, exhibited changes at the transcriptional level and jointly allowed the strain to protect against environmental stresses.

Highlights

Sphingomonas sp. is a promising producer of gellan gum polysaccharide, such as gellan gum, welan gum, rhamsan gum, and so on
Stress-resistant elements derived from extremophiles, including heat shock proteins GroES of Thermoanaerobacter tengcongensi, GroEL of Thermus thermophilus and global transcriptional regulator IrrE of Deinococcus radiodurans, were heterologously expressed in the welan gum-producing strain S. sp
When the fermentation temperature increased to 40◦C, the recombinant strains S. sp.-pBBR-groes and S. sp.-pBBR-irre showed significant advantage compared with the control strains, the biomass and welan gum concentration at the end of fermentation process were slightly lower than those of 37◦C

Summary

Introduction

Sphingomonas sp. is a promising producer of gellan gum polysaccharide, such as gellan gum, welan gum, rhamsan gum, and so on. Heat from mechanical agitation and microbial metabolism (Zhu et al, 2014a), high viscosity and weak acidity from welan gum accumulation (Li et al, 2011a), and pigment from carotenoid synthesis (Zhang et al, 2016) result in the large demand of cooling water, alkali binding agent, strong ventilation or agitation for fermentation, and alcohol for polysaccharide purification, thereby causing high energy consumption and cost. By atmospheric and room-temperature plasma-induced mutation, and the mutant could grow well at 37◦C. These approaches have difficult in mutation direction grasping, beneficial combinatorial landscape occurrence, target strain screening, and good traits passing on (Blooma and Arnoldb, 2009). Current metabolic engineering approaches are limited by multiple gene modification (Alper et al, 2005)

Methods

Results

Discussion

Conclusion