Studies of the biosynthesis of the nitro sugar D-kijanose and the function of the glycosyltransferase helper proteins in glycosylation of macrolide antibiotics
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The appended sugar residues of many natural products from Actinomyces are important for their biological activities. Many of these unusual sugar biosynthetic gene clusters have been isolated and many glycosyltransferases from various antibiotic-producing organisms have been identified. The increasing knowledge about these sugar biosynthetic pathways opens up the possibility of generating novel bioactive glycosylated compounds through combinatorial biosynthesis. The work described in this dissertation focuses on the investigation of the biosynthetic pathway of a rare nitro-containing sugar, D-kijanose, from an antibiotic, kijanimicin, and the glycosyltransferase helper proteins involved in the glycosylation of macrolide antibiotics. D-Kijanose, especially its nitro group, plays an important role in conferring the biological activities of the parent antibiotics. Cloning and sequencing of the kijanimicin biosynthetic gene cluster have allowed the proposal of the biosynthetic pathway of D-kijanose. The functions of the enzymes encoded by each open-reading frame in the cluster were also assigned based on sequence comparison with known enzymes found in other biosynthetic reactions. In this thesis, the functions of KijB1, a TDP-4-keto-6-deoxy-hexose 2,3-dehydratase, and KijD2, a TDP-hexose C-3 aminotransferase, were verified. The TDP-3-amino-4-keto-2,3,6-trideoxyhexose produced as an intermediate in the early stage of D-kijanose biosynthesis was also identified. In the second part of this dissertation, the in vivo protein-protein interaction between D-desosaminyl glycosyltransferase, DesVII, and its auxiliary protein, DesVIII, was established by yeast two-hybrid assay. The complex formation between these two proteins was also demonstrated by in vitro binding assay. Several strategies were tried to overexpress the D-mycaminosyl glycosyltransferase and its auxiliary protein, TylM2 and TylM3, although none of them were successful. A two-plasmid in vivo glycosylation system was also developed to test the competence of various DesVIII homologues to serve as the helper protein for glycosyltransferase DesVII, MycB and NbmD. In summary, the work in this dissertation has provided important information on the biosynthesis of D-kijanose and also significant insight into the function of the helper proteins of macrolide glycosyltransferases. These results could be useful for future studies of natural product biosynthesis and exploitation of glycodiversification.