Phenolic aldehydes from lignocellulose pretreatment harshly inhibit the viability and metabolism of ethanol fermenting strains. Direct conversion of phenolic aldehydes is usually incomplete due to their low water solubility and recalcitrance to bioconversion. Here we consolidated phenolic aldehydes bioconversion and ethanol fermentation in a typical ethanologenic bacterium Zymomonas mobilis by constructing an intracellular oxidative pathway. The gene PP_2680 encoding NAD + -dependent aldehyde dehydrogenase from Pseudomonas putida KT2440 was expressed in Z. mobilis ZM4. The expression significantly improved both aldehyde inhibitor conversion and ethanol fermentability in corn stover hydrolysate. The purified PP_2680 aldehyde dehydrogenase showed strong in vitro oxidative capacity on phenolic aldehydes and its in vivo expression significantly up-regulated the key genes in the ED pathway and the oxidative phosphorylation. This study provided an important concept of simultaneous biodetoxification and fermentation in ethanologenic strains for the improvement of ethanol fermentability.
Kinases modulate the various physiological activities of microbial fermenting strains including the conversion of lignocellulose-derived phenolic aldehydes (4-hydroxyaldehyde, vanillin, and syringaldehyde). Here, we comprehensively investigated the gene transcriptional profiling of the kinases under the stress of phenolic aldehydes for ethanologenic Zymomonas mobilis using DNA microarray. Among 47 kinase genes, three genes of ZMO0003 (adenylylsulfate kinase), ZMO1162 (histidine kinase), and ZMO1391 (diacylglycerol kinase), were differentially expressed against 4-hydroxybenzaldehyde and vanillin, in which the overexpression of ZMO1162 promoted the phenolic aldehydes conversion and ethanol fermentability. The perturbance originated from plasmid-based expression of ZMO1162 gene contributed to a unique expression profiling of genome-encoding genes under all three phenolic aldehydes stress. Differentially expressed ribosome genes were predicted as one of the main contributors to phenolic aldehydes conversion and thus finally enhanced ethanol fermentability for Z. mobilis ZM4. The results provided an insight into the kinases on regulation of phenolic aldehydes conversion and ethanol fermentability for Z. mobilis ZM4, as well as the target object for rational design of robust biorefinery strains.
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