Pleiotropic Global Regulator Lrp of Free-Living Nitrogen-Fixing Ethanologenic Bacterium Couples Carbon–Nitrogen Metabolism

Rational
design and construction of robust microbial cell factories
(MCFs) require comprehensive information about metabolic and regulatory
networks. However, this is hindered by limited information and toolkits
for nonmodel industrial microorganisms. Here, we investigated the
function of pleiotropic global regulator Lrp in the nitrogen-fixing
ethanologenic bacterium Zymomonas mobilis. Although the deletion of lrp significantly reduced
cell growth under both feast and famine conditions, the nitrogen fixation
efficiency was enhanced. This could be attributed to the upregulation
of nitrogen fixation regulators nifA and glnB in Δlrp based on omics analyses and electrophoretic
mobility shift assay results. Additionally, Lrp exhibits a broad regulatory
role in regulating genes involved in nitrogen and carbon metabolism,
including those associated with ammonia assimilation, amino acid metabolism,
the TCA cycle, and one-carbon metabolism. Moreover, independent, concerted,
and reciprocal regulatory modes of Lrp upon leucine supplementation
were proposed. Notably, lrp deletion was also confirmed
to effectively enhance nitrogen fixation capability in a d-lactate-producing Z. mobilis strain
and the model legume-rhizobia symbiotic bacterium Mesorhizobium
loti. This study thus provides new insights into Lrp-mediated
regulation of nitrogen fixation and carbon–nitrogen metabolisms
as well as strategies to release the tightly controlled nitrogen fixation,
which contributes to the design and construction of robust and efficient
MCFs.