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Background Extremolytes enable microbes to withstand even the most extreme conditions in nature. Due to their unique protective properties, the small organic molecules, more and more, become high-value active ingredients for the cosmetics and the pharmaceutical industries. While ectoine, the industrial extremolyte flagship, has been successfully commercialized before, an economically viable route to its highly interesting derivative 5-hydroxyectoine (hydroxyectoine) is not existing. Results Here , we demonstrate high-level hydroxyectoine production, using metabolically engineered strains of C. glutamicum that express a codon-optimized, heterologous ectD gene, encoding for ectoine hydroxylase, to convert supplemented ectoine in the presence of sucrose as growth substrate into the desired derivative. Fourteen out of sixteen codon-optimized ectD variants from phylogenetically diverse bacterial and archaeal donors enabled hydroxyectoine production, showing the strategy to work almost regardless of the origin of the gene. The genes from Pseudomonas stutzeri (PST) and Mycobacterium smegmatis (MSM) worked best and enabled hydroxyectoine production up to 97% yield. Metabolic analyses revealed high enrichment of the ectoines inside the cells, which, inter alia , reduced the synthesis of other compatible solutes, including proline and trehalose. After further optimization, C.   glutamicum Ptuf ectD PST achieved a titre of 74 g L −1 hydroxyectoine at 70% selectivity within 12 h, using a simple batch process. In a two-step procedure, hydroxyectoine production from ectoine, previously synthesized fermentatively with C. glutamicum ectABC opt , was successfully achieved without intermediate purification. Conclusions C. glutamicum is a well-known and industrially proven host, allowing the synthesis of commercial products with granted GRAS status, a great benefit for a safe production of hydroxyectoine as active ingredient for cosmetic and pharmaceutical applications. Because ectoine is already available at commercial scale, its use as precursor appears straightforward. In the future, two-step processes might provide hydroxyectoine de novo from sugar.

The compatible solute ectoine is known to attenuate inflammatory effects in the airways after exposure to combustion-derived nanoparticles. Pro-inflammatory signaling in epithelial cells, as well as antiapoptotic mechanisms in neutrophilic granulocytes, both triggered by particles, are reduced by this substance. Here we investigated the preventive potential in airway inflammation of additional compounds originating from the ectoine metabolism, Nγ-acetyl-L-2,4-diaminobutyric acid (NADA), and 5-hydroxyectoine in a mouse model and in human neutrophilic granulocytes. Furthermore, effects of these molecules on the reduction in cystic fibrosis transmembrane conductance regulator (CFTR), as an additional pathogenic endpoint of nanoparticle exposure, were investigated. All three solutes exhibited beneficial effects at the level of inflammatory cells in lung lavages from exposed mice. The decrease in CFTR in lung tissue of exposed mice was mitigated by the substances. In primary human neutrophils and in neutrophilic differentiated HL-60 cells, the delay of apoptosis rates after particle exposure was effectively abolished. The decline in CFTR from the cytoplasmic membrane in neutrophilic cells was also counteracted by the compatible solutes. The data identify both NADA and 5-hydroxyectoine as additional substances for molecular prevention of airway effects of environmental particles. Furthermore, the reduction in CFTR might be a relevant finding for patients suffering from impaired function of this ion channel.