Characterization of Streptomyces nogalater genes encoding enzymes involved in glycosylation steps in nogalamycin biosynthesis

The sno gene cluster in Streptomyces nogalater ATCC 27451 contains the nogalamycin biosynthesis genes. A set of plasmid constructions carrying fragments of the sno cluster that lie downstream of snoD were used to complement the S. galilaeus mutant H039, which is blocked in rhodosamine and 2-deoxyfucose biosynthesis in the aclacinomycin pathway. Sequence analysis of this cluster revealed three contiguous open reading frames (ORFs) that were designated snoF, snoG, and snoH. Only those plasmid constructs that expressed SnoG were able to complement H039. SnoG shows similarity to GalE, a UDP-glucose-4-epimerase catalyzing the epimerization of UDP-glucose to UDP-galactose. The putative SnoF protein is similar to 3,5-epimerases involved in rhamnose biosynthesis. The deduced product of snoH is a 489-amino acid polypeptide. It is similar to the product of dau ORF3 found in the daunomycin cluster. However its function is still unclear. Based on the complementation experiments and sequence analysis, this part of the size cluster is suggested to be involved in the biosynthesis of the sugar portion of nogalamycin. Interestingly, SnoA, a transcriptional activator for the sno minimal polyketide synthase, is also needed to express this cluster.