Glycopeptide antibiotics have long served as drugs of last resort for the treatment of antibiotic-resistant Gram-positive bacterial infections. Resistance to the clinically relevant glycopeptides, vancomycin and teicoplanin, threatens to undermine the usefulness of this important class of antibiotics. DNA extracted from a geographically diverse collection of soil samples was screened by PCR for the presence of sequences related to OxyC, an oxidative coupling enzyme found in glycopeptide biosynthetic gene clusters. Every soil sample examined contained at least 1 unique OxyC gene sequence. In an attempt to access the biosynthetic gene clusters associated with these OxyC sequences, a 10,000,000-membered environmental DNA (eDNA) megalibrary was created from a single soil sample. Two unique glycopeptide gene clusters were recovered from this eDNA megalibrary. Using the teicoplanin aglycone and the 3 sulfotransferases found in one of these gene clusters, mono-, di-, and trisulfated glycopeptide congeners were produced. The high frequency with which OxyC genes were found in environmental samples indicates that soil eDNA libraries are likely to be a rewarding source of glycopeptide gene clusters. Enzymes found in these gene clusters should be useful for generating new glycopeptides analogs. Environmental DNA megalibraries, like the one constructed for this study, can provide access to many of the natural product biosynthetic gene clusters that are predicted to be present in soil microbiomes.antibiotics ͉ eDNA ͉ metagenomics ͉ natural products ͉ uncultured bacteria
Cysteine dioxygenase (CDO, EC 1.13.11.20) catalyzes the oxidation of cysteine to cysteine sulfinic acid, which is the first major step in cysteine catabolism in mammalian tissues. Rat liver CDO was cloned and expressed in Escherichia coli as a 26.8-kDa N-terminal fusion protein bearing a polyhistidine tag. Purification by immobilized metal affinity chromatography yielded homogeneous protein, which was catalytically active even in the absence of the secondary protein-A, which has been reported to be essential for activity in partially purified native preparations. As compared with those existing purification protocols for native CDO, the milder conditions used in the isolation of the recombinant CDO allowed a more controlled study of the properties and activity of CDO, clarifying conflicting findings in the literature. Apo-protein was inactive in catalysis and was only activated by iron. Metal analysis of purified recombinant protein indicated that only 10% of the protein contained iron and that the iron was loosely bound to the protein. Kinetic studies showed that the recombinant enzyme displayed a K m value of 2.5 ؎ 0.4 mM at pH 7.5 and 37°C. The enzyme was shown to be specific for L-cysteine oxidation, whereas homocysteine inhibited CDO activity.
A detailed bioinformatics analysis of six glycopeptide biosynthetic gene clusters isolated from soil environmental DNA (eDNA) mega-libraries indicates that a subset of these gene clusters contains collections of tailoring enzymes that are predicted to result in the production of new glycopeptide congeners. In particular, sulfotransferases appear in eDNA derived gene clusters at a much higher frequency than would be predicted from the characterization of glycopeptides from cultured Actinomycetes. Enzymes found on tailoring enzyme-rich eDNA clones associated with these six gene clusters were used to produce a series of new sulfated glycopeptide derivatives in both in vitro and in vivo derivatization studies. The derivatization of known natural products with eDNA derived tailoring enzymes is likely to be a broadly applicable strategy for generating libraries of new natural product variants.
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