title: Threonine aldolase overexpression plus threonine supplementation enhanced riboflavin production in Ashbya gossypii.
abstract: Riboflavin production in the filamentous fungus Ashbya gossypii is limited by glycine, an early precursor required for purine synthesis. We report an improvement of riboflavin production in this fungus by overexpression of the glycine biosynthetic enzyme threonine aldolase. The GLY1 gene encoding the threonine aldolase of A. gossypii was isolated by heterologous complementation of the glycine-auxotrophic Saccharomyces cerevisiae strain YM13 with a genomic library from A. gossypii. The deduced amino acid sequence of GLY1 showed 88% similarity to threonine aldolase from S. cerevisiae. In the presence of the GLY1 gene, 25 mU of threonine aldolase specific activity mg-1 was detectable in crude extracts of S. cerevisiae YM13. Disruption of GLY1 led to a complete loss of threonine aldolase activity in A. gossypii crude extracts, but growth of and riboflavin production by the knockout mutant were not affected. This indicated a minor role of the enzyme in glycine biosynthesis of A. gossypii. However, overexpression of GLY1 under the control of the constitutive TEF promoter and terminator led to a 10-fold increase of threonine aldolase specific activity in crude extracts along with a 9-fold increase of riboflavin production when the medium was supplemented with threonine. This strong enhancement, which could not be achieved by supplementation with glycine alone, was attributed to an almost quantitative uptake of threonine and its intracellular conversion into glycine. This became evident by a subsequent partial efflux of the glycine formed.
authors.affiliation: Institut für Biotechnologie 1, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Monschau
authors.forename: H
authors.initials: H
authors.lastname: Sahm
authors.forename: K
authors.initials: K
authors.lastname: Stahmann
mesh_terms: D000595:Amino Acid Sequence; D001203:Ascomycota / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D003001:Cloning, Molecular; D003470:Culture Media; D004926:Escherichia coli; D005821:Genetic Techniques; D015698:Genomic Library; D012696:Glycine Hydroxymethyltransferase / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics*; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011995:Recombination, Genetic; D012256:Riboflavin / Q000096:biosynthesis*; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013912:Threonine / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D003470:Culture Media; D013912:Threonine; D012696:Glycine Hydroxymethyltransferase; D012256:Riboflavin