b-NAD as a Building Block in Natural Product Biosynthesis

b-Nicotinamide adenine dinucleotide (b-NAD) is a pivotal metabolite for all living organisms and functions as a primary electron acceptor and carrier during central catabolic processes. In this study, we demonstrate the first example that b-NAD functions as a building block for the assembly of potent and structurally intriguing bacterial secondary metabolites, such as the anti-cancer compound altemicidin and the Ile-tRNA inhibitor SB-203208 from Streptomyces. Our biochemical analyses identified a PLP-dependent enzyme (SbzP) as a novel family of enzyme catalyzing the scaffold formation via [2+3]-annulation reaction at the pyridinium moiety of b-NAD, utilizing S-adenosyl methionine as a co-substrate. Reconstitution of the complete downstream biosynthetic pathway revealed the function of several new dinucleotide-tailoring enzymes, leading to altemicidin scaffold. Intriguingly, SbzP homologs are distriubted among various genomes of Gram-positive and -negative bacterial phyla, such as actino-, chloroflexi-, proteo- and cyanobacteria, and b-NAD-utilization was demonstrated for eight representative proteins by heterologous expression. The findings of this study fundamentally expand our understanding of the bacterial secondary metabolic repertoire and will enable the discovery and exploitation of a novel class of b-NAD-derived natural products.