Improved xylose tolerance and 2,3-butanediol production of Klebsiella pneumoniae by directed evolution of rpoD and the mechanisms revealed by transcriptomics.

Background: The biological production of 2,3-butanediol from xylose-rich raw materials from Klebsiella pneumoniae is a low-cost process. RpoD , an encoding gene of the sigma factor, is the key element in global transcription machinery engineering and has been successfully used to improve the fermentation with Escherichia coli . However, whether it can regulate the tolerance in K. pneumoniae remains unclear.

Results: In this study, the kpC mutant strain was constructed by altering the expression quantity and genotype of the rpoD gene, and this exhibited high xylose tolerance and 2,3-butanediol production. The xylose tolerance of kpC strain was increased from 75 to 125 g/L, and the yield of 2,3-butanediol increased by 228.5% compared with the parent strain kpG, reaching 38.6 g/L at 62 h. The RNA sequencing results showed an upregulated expression level of 500 genes and downregulated expression level of 174 genes in the kpC mutant strain. The pathway analysis further showed that the differentially expressed genes were mainly related to signal transduction, membrane transport, carbohydrate metabolism, and energy metabolism. The nine most-promising genes were selected based on transcriptome sequencing, and were evaluated for their effects on xylose tolerance. The overexpression of the tktA encoding transketolase, pntA encoding NAD(P) transhydrogenase subunit alpha, and nuoF encoding NADH dehydrogenase subunit F conferred increased xylose consumption and increased 2,3-butanediol production to K. pneumoniae .

Conclusions: These results suggest that the xylose tolerance and 2,3-butanediol production of K. pneumoniae can be greatly improved by the directed evolution of rpoD. By applying transcriptomic analysis, the upregulation of tktA , pntA , and nuoF that were coded are essential for the xylose consumption and 2,3-butanediol production. This study will provide reference for further research on improving the fermentation abilities by means of other organisms.