Metabolic engineering of Streptomyces albulus for de novo serotonin production

Serotonin (5-hydroxytryptamine, 5-HT) is a pivotal neurotransmitter with broad clinical relevance. However, its industrial-scale microbial production remains a significant challenge. In this study, we engineered Streptomyces albulus to develop an efficient enzymatic cascade for the scalable and high-yield biosynthesis of 5-HT. To enhance 5-HT production, we first overexpressed the native 5-hydroxytryptophan decarboxylase (5-OHTDC) in S. albulus, which strengthened the final decarboxylation step. Using 5-hydroxytryptophan as the substrate, this modification achieved a 5-HT titer of 6.87 g/L. We then introduced Escherichia coli L-tryptophan permease (Mtr) to improve cellular uptake of L-tryptophan, along with Actinomadura luzonensis tryptophan 5-hydroxylase (Luz15) to catalyze the hydroxylation of L-tryptophan. This triple-enzyme expression system (5-OHTDC/Mtr/Luz15) enabled the direct conversion of L-tryptophan to 5-HT, yielding 3.46 g/L. Leveraging the innate L-tryptophan biosynthesis capability of S. albulus, we pursued direct fermentative production from low-cost carbon sources. Mannitol was identified as the optimal substrate, supporting a maximum 5-HT titer of 6.54 g/L in fed-batch fermentation. Finally, implementing an L-tryptophan feeding strategy further boosted the final 5-HT titer to 12.0 g/L. This work establishes S. albulus as an efficient microbial platform for high-level 5-HT production. The engineered strain and optimized fermentation process demonstrate strong potential for industrial application and provide a versatile chassis for synthesizing diverse tryptophan-derived bioactive compounds.