Analysis of the Mildiomycin Biosynthesis Gene Cluster in Streptoverticillum remofaciens ZJU5119 and Characterization of MilC, a Hydroxymethyl cytosyl-glucuronic Acid Synthase

Mildiomycin (MIL) is a peptidyl-nucleoside antibiotic produced by Streptoverticillum remofaciens ZJU5119 that exhibits strong inhibitory activity against powdery mildew. The entire MIL biosynthesis gene cluster was cloned and expressed in Streptomyces lividans 1326. Systematic gene disruptions narrowed down the cluster to 16 functional ORFs and identified the boundaries of the gene cluster. A putative cytosylglucuronic acid (CGA) synthase gene, milC, was disrupted in Sv. remofaciens and heterologously expressed in E. coli. An in vitro assay revealed that purified MilC could utilize either cytosine or hydroxymethylcytosine as substrate to yield CGA or hydroxymethyl-CGA (HM-CGA), respectively. MilG is believed to be a key enzyme in the MIL biosynthesis pathway and contains the C_XXXC_XXC motif characteristic of members of the radical S-adenosyl methionine (SAM) superfamily. Disruption of milG leads to accumulation of HM-CGA. Labeling experiments with ¹³C₆-L-arginine indicated that decarboxylation at C5 of the pyranoside ring was coupled with the attachment of 5-guanidino-2,4-dihydroxyvalerate side chain through C-C bond formation. In contrast, exogenous ¹³C₆-labeled 4-hydroxy-L-arginine was not incorporated into the MIL structure. Comparative analysis of the 16 MIL ORFs with counterparts involved in the biosynthesis of the structurally similar compound blasticidin S, along with the results above, provide insight into the complete MIL biosynthetic pathway.