Discovery and characterization of the tubercidin biosynthetic pathway from Streptomyces tubercidicus NBRC 13090

Background
Tubercidin (TBN), an adenosine analog with potent antimycobacteria and antitumor bioactivities, highlights an intriguing structure, in which a 7-deazapurine core is linked to the ribose moiety by an N-glycosidic bond. However, the molecular logic underlying the biosynthesis of this antibiotic has remained poorly understood.

Results
Here, we report the discovery and characterization of the TBN biosynthetic pathway from Streptomyces tubercidicus NBRC 13090 via reconstitution of its production in a heterologous host. We demonstrated that TubE specifically utilizes phosphoribosylpyrophosphate and 7-carboxy-7-deazaguanine for the precise construction of the deazapurine nucleoside scaffold. Moreover, we provided biochemical evidence that TubD functions as an NADPH-dependent reductase, catalyzing irreversible reductive deamination. Finally, we verified that TubG acts as a Nudix hydrolase, preferring Co2+ for the maintenance of maximal activity, and is responsible for the tailoring hydrolysis step leading to TBN.

Conclusions
These findings lay a foundation for the rational generation of TBN analogs through synthetic biology strategy, and also open the way for the target-directed search of TBN-related antibiotics.