d-amino acid auxotrophic Escherichia coli strain for in vivo functional cloning of novel d-amino acid synthetic enzyme.

Various d-amino acids have been found in a wide range of organisms, including mammals. Although the physiological functions of various d-amino acids have been reported or suggested, the molecular basis of these biological functions has been elucidated in only a few cases. The identification of a d-amino acid biosynthetic enzyme is a critical step in understanding the mechanism of the physiological functions of d-amino acids. While in vivo functional screening can be a powerful tool for identifying novel metabolic enzymes, none of the existing organisms exhibit growth dependent on d-amino acid other than d-Ala and d-Glu. Here, we report the first organism that exhibits non-canonical d-amino acid auxotrophy. We found that an Escherichia coli strain lacking the major d-Ala and d-Glu biosynthetic enzymes, alr, dadX, and murI, and expressing the mutated d-amino acid transaminase (DAAT) gene from Bacillus sp. YM-1 (MB3000/mdaat + ) grew well when supplemented with certain d-amino acid. A multicopy suppression study with plasmids encoding one of the 51 PLP-dependent enzymes of E. coli showed that MB3000/mdaat + could detect weak and moonlighting racemase activity, such from cystathionine β-lyase (MetC) and a negative regulator of MalT activity/cystathionine β-lyase (MalY)-these exhibit only a few tenths to a few thousandths of the racemization activity of canonical amino acid racemases. We believe that this unique platform will contribute to further research in this field by identifying novel d-amino acid-metabolizing enzymes.