As a typical representative of thiopeptide antibiotics, nosiheptide (NOS) possesses very good antibacterial activity. However, due to poor water solubility and low bioavailability, its clinical application is hampered. Due to its complex structure, it is difficult to obtain analogues with improved physical and chemical properties via total chemical synthesis. Based on the previous studies on the biosynthesis of nosiheptide, the side-ring 3-methyl-2-indoleic acid (MIA) analogues were used as chemical small molecule probes to explore the substrate tolerance of enzymes involved in NOS biosynthesis pathway in NOS-producing bacteria via the co-fermentation of probe molecules with mutant strain and the combination of high resolution mass spectrometry data of fermentation products. The results showed that enzymes involved in NOS biosynthesis pathway had a considerable tolerance to MIA analogues substituted by F, Cl and CH₃, however, MIA analogues substituted by large steric hindrance group, such as NO₂, CF₃ and Ph, were not tolerated. The position, the size and the property of the substituted groups of MIA also affected the steps of identification, transport and upload of the related enzymes involved in NOS biosynthesis. The present study not only explored the substrate tolerance of enzymes involved in NOS biosynthesis pathway, but also was expected to obtain NOS analogues via biosynthetic pathway engineering. What’s more, it provides valuable information for using directed evolution technology to improve the substrate tolerance of enzymes in the rate-limiting steps of NOS biosynthesis and to expand the use of NOS-producing bacteria to obtain more analogues.

Key words: nosiheptide, 3-methyl-2-indolic acid, mutational biosynthesis, chemical small molecule probe, substrate tolerance of enzymes