Genetically engineered hybrid strains, in which the genes of the urdamycin‐producer Streptomyce fradiae are combined with some of the biosynthetic genes of the elloramycin‐producer Streptomyces olivaceus or the genes of the tetracenomycin‐producer Streptomyces glaucescens, assemble novel hybrid antibiotics of the tetracenomycin type. The results prove that this combinatorial biosynthesis method may be a useful alternative for the production of new natural products which can then be screened.
Cosmid 16F4 contains 25 kb of the elloramycin biosynthetic pathway of Streptomyces olivaceus
Tü2353. Transformation of this cosmid into a polyketide synthase (PKS)-deleted mutant of the urdamycin
producer, Streptomyces
fradiae Tü2717/ΔPKS and into the mithramycin producer Streptomyces
argillaceus
ATCC 12956 resulted in the production of several novel glycosylated tetracenomycins. Four of the structures
of these elloramycin analogues (3, 5−7) were elucidated. They carry various deoxysugar moieties (d-olivose,
l-rhodinose, d-mycarose, and a disaccharide consisting of two 1,3-linked d-olivoses) attached at C-8-O of the
same aglycon, 8-demethyltetracenomycin C (4). The transfer of the sugars is not catalyzed by glycosyltransferases of the S.
fradiae or S.
argillaceus strains since the novel hybrid tetracenomycins are also produced by
a S.
argillaceus mutant carrying cosmid 16F4 but lacking all the known mithramycin glycosyltransferases.
Furthermore, a Streptomyces
lividans strain containing cosmid 16F4 produced the novel tetracenomycins only
when a second plasmid containing the cloned mithramycin sugar biosynthetic genes but lacking glycosyltransferase genes was also present. The glycosyl transfer therefore must be catalyzed by an elloramycin
glycosyltransferase encoded by cosmid 16F4. Apparently, this glycosyltransferase is able to catalyze the
glycosylation of 8-demethyltetracenomycin C (4, = 12a-demethylelloramycinone) using various d- and l-sugars
including a disaccharide. Its future use for combinatorial biosynthetic approaches is discussed.
In order to investigate mid and early biosynthetic steps of angucycline group antibiotics, approximately 400 mutants of the urdamycin producer Streptomyces fradiae (strain Tii 2717) were prepared, of which ca. 10 76 were selected for further investigations. The selection criterion, i.e., the consideration of only pale-colored metabolite-producing blocked mutants, yielded several mutants whose block was in close proximity to the known late-stage biosynthetic steps. The product patterns were characterized by TLC and HPLC methods, and the structures of five new and one known (but previously not detectable) metabolites were elucidated (3)(4)(5)(6)(7)(8). Their roles in the biosynthetic pathway leading to aquayamycin (1) and on to the urdamycins A (2) and B (9) are proposed. The glycosylation sequence of the urdamycin group and two additional earlier biosynthetic steps leading to aquayamycin (l), the most important angucyclinone, were established in this way.
Water and not acetate is the source of the oxygen atom 4a‐OH in the biosynthesis of tetracenomycin C. Based on this unusual finding, an intermediate epoxide is postulated, which has to be “cis opened” to attain the required stereochemistry at C‐4a and C‐12a (see reaction sequence below).
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