Biosynthesis and enzymology of azaindane natural products

As a representative example of biosynthetic genome mining aimed at identifying biosynthetic gene clusters whose target is unknown core enzymes for known natural products, we review the biosynthesis of altemicidin (1), SB-203207 (2), and SB-203208 (3). Self-resistance gene-guided genome mining led to the identification of the responsible biosynthetic gene cluster, and heterologous expression of the cluster successfully confirmed the production of compounds 1-3. Biochemical analyses and single-gene expression studies demonstrated that the PLP-dependent enzyme SbzP is a core enzyme of this gene cluster. SbzP accepts β-NAD and SAM as substrates to generate the azaindane scaffold common to compounds 1-3. Subsequent in vitro assays revealed the downstream tailoring reactions, involving the α-ketoglutarate-dependent dioxygenase SbzQ, GNAT-type acyltransferase SbzI, ADP-ribose transferases SbzNHO, F420-dependent reductase SbzF, SAM-dependent methyltransferase SbzE, acyl-tRNA-dependent transferase SbzA, and GNAT-type acyltransferase SbzC, which collectively modify the SbzP product to yield the final structure of compound 3. Structural studies of SbzP and SbzI elucidated the molecular basis for substrate recognition and protein-protein interactions. This review highlights the novelty and utility of investigating biosynthetic gene clusters categorized as Group III and provides new insights into the biosynthesis of β-NAD-derived natural products.