In vivo reconstitution of the dTDP-D-desosamine pathway of the megalomicin gene cluster from Micromonospora megalomicea was achieved by expression of the genes in Escherichia coli. LC/MS/MS analysis of the dTDP-sugar intermediates produced by operons containing different sets of genes showed that production of dTDP-D-desosamine from dtdp-4-keto-6-deoxy-D-glucose requires only four biosynthetic steps, catalysed by MegCIV, MegCV, MegDII and MegDIII, and that MegCII is not involved. Instead, bioconversion studies demonstrated that MegCII is needed together with MegCIII to catalyse transfer of D-desosamine to 3-alpha-mycarosylerythronolide B.