Metabolic engineering of Ashbya gossypii for enhanced FAD production through promoter replacement of FMN1 gene

Riboflavin (vitamin B2), Flavin Mononucleotide (FMN), Flavin Adenine Dinucleotide (FAD) are essential biomolecules for carrying out various metabolic activities of oxidoreductases and other enzymes. Riboflavin is mainly used as food and feed supplement while the more expensive FAD has pharmacological importance. Although Ashbya gossypii has been metabolically engineered for industrial production of riboflavin, there are no reports on FAD production. In the present study, a transcriptional analysis of the time course of flavin genes expression, indicated that riboflavin to FMN conversion by riboflavin kinase enzyme encoded by FMN1 gene could be the major rate limiting step in FAD synthesis. Overexpression of FMN1 gene was attempted by placing the ORF of FMN1 under control of the stronger constitutively expressed GPD (Glyceraldehyde-3-phosphate dehydrogenase) promoter replacing its native promoter. A 2.25Kb promoter replacement cassette (PRC) for FMN1 gene was synthesized from cloned pUG6-GPDp vector and used for transformation of Ashbya gossypii. Resultant recombinant strain CSAgFMN1 had 35.67-fold increase in riboflavin kinase enzyme activity. A 14.02-fold increase in FAD production up to 86.56 ± 3.88 mg L−1 at 120 h incubation was obtained compared to wild type. While there was a marginal increase in riboflavin synthesis by the clone, FMN accumulation was not detected and could be attributed to other metabolic fluxes channeling FMN. This is the first report on development of FAD overproducing strain of A. gossypii.