Blocked mutants

Macrolides containing only neutral sugars were obtained from a platenomycin-producing organism (217,218). Four demycarosyl derivatives of platenomycins were isolated from biosyntheticaHy blocked mutants of S. platensis, two of these possessed a methyl group rather than an aldehyde (219). A pair of novel compounds related to carbomycin were isolated in which one contained an unusual 10,ll-dihydro-12,13-diol moiety, the other a 14-hydroxy-epoxyenone moiety (220). 

The proposed pathway for the biosynthesis of the avermectins (Fig. 3) has been described in a review (23). Some of the details are yet to be elucidated, although the steps, in general, are based on firm evidence from four types of studies incorporation of labeled precursors, conversion of putative intermediates by producing strains and blocked mutants, in vitro measurement of biosynthetic enzymes, and studies with enzyme inhibitors. The biosynthesis of the oleandrose units was elucidated from studies using and labeled glucose, which indicated a direct conversion of glucose to... 

Deoxyerythronolide B (28), produced by blocked mutants of Streptomyces erythreus, is a common biosynthetic precursor leading to erythromycins. A different route to this compound was developed with aldol methodology.5 In this approach, all the crucial C C bond formations involved in the construction of the carbon framework are exclusively aldol reactions. 

Microorganisms are able to carry out different reactions in which a compound is converted into a structurally related product. These processes, which are catalyzed by one or several enzymes of cells, are stereospecific. Reaction conditions are mild, therefore convenient, and in many cases are preferable to chemical routes. The concentration of the desired products may be increased by the selection of properly blocked mutants. In Table 1 are listed some of many types of reactions that have been carried out with microorganisms. The product yields are usually high (Table 2). 

Emycin C (215) and the oxygen-rich emycin H (216) are novel minor congeners of a blocked mutant of the emycin producer Streptomyces cellulusae ssp. griseoincarnatus (Scheme 51) (mutant 1114-2 see also Sects. 9.5 and 10.2) [155-157]. 

Compound 100-2 (241, 12b-L-rhodinosylaquayamycin), isolated from a blocked mutant of the urdamycin producer Streptomyces fradiae Til 2717, represents the missing link in the glycosylation sequence of the urdamycins [164]. Structure 241 proves that the linkage of the L-rhodinose at C-12b-0 is the first of three 0-glycosylation steps (after the C-glycosylation of one olivose several biosynthetic steps before, see Sect. 10). 

Table 14. Momofulvenones A.B (272,271) and Tii960R (273), a natural product from a blocked mutant of the lysolipin I (274) producer Streptomyces violaceoniger Tii9...

Thus from an analysis of the biosynthetic gene clusters as well as from earlier biosynthetic studies (often with blocked mutants), it is useful to divide the poly-ketide biosynthetic pathway into PKS reactions and post-PKS tailoring steps. Several of the latter were investigated with respect to biosyntheses of angucy-clin(on)es and natural products derived thereof. Intensive studies of various blocked mutant products of Streptomyces fradiae Til 2717 resulted in a well established sequence of the late steps of urdamycin A (281) biosynthesis (Scheme 63) [164]. 

Dioxygenases present in the blocked mutants of Pseudomonas putida, a soil bacterium, degrades benzene and its derivatives into cyclohexa-3,5-diene-l,2-diols. With chlorobenzene, diol 387 is obtained with > 99% ee. This compound is converted in a few chemical steps into... 

R)-Camitine formation by a dehydrogenase-blocked mutant Agrobacterium strain ... 

Using a blocked mutant of the erythromycin-producing Streptomyoes erythreus, biotransformation of lankamycln, darcanollde and 11-acetyl-lankolide was performed, but the products showed poor activity. ... 

Hendrickson, L., Davis, C. R., Roach, C. D., Nguyen, K., Aldrich McAda, T. P. C., and Reeves, C. D. 1999. Lovastatin biosynthesis in Aspergillus terreus Characterization of blocked mutants, enzyme activities and a multifunctional polyketide synthase gene. Chem. Biol. 6, 429-439. 

Another fortimicin analog is sporaricin E or 2-deoxy fortimicin B, which shows weak antibacterial activity [112]. In a report by Morioka et al. it was demonstrated that a blocked mutant of istamycin producing Streptomyces tenjimariensis converts fortimicin B to l-c/ i-dactimicin with inversion of the amino group at position 1 and N-4-acylation [113]. 

S. coerulerorubidus produces different daunomycin (35) derivatives. Various blocked mutants have been prepared by UV and chemical mutation. One of these mutants produced the anthracyclines feudomycin A (36) and B (37) which were not produced by the wild-type strain. The structure of the new compounds indicated that the starter carbon units in the formation of these new compounds were acetate, butyrate, and acetoacetate instead of propionate which is used by the native proteins [26] (Fig. 12.10). Similar results have been obtained in others studies [27,28] (see also Section 12.3.5.1.2). 

Figure 12.10. Structures of daunomycin (35)-derivalives (36,37) produced by blocked mutants of S. coerulerorubidus.

Hegeman, G.D. 1966b. Synthesis of the enzymes of the mandelate pathway by Pseudomonas putida. II. Isolation and properties of blocked mutants. J. Bacteriol. 91 1155-1160. 

Relomycin (Table 7) was found in culture broths of S. hygroscopicus (242) and shown to be 20-dihydrotylosin by chemical and microbial reductions (242,243). Macrocin (3 -0-demethyltylosin), was found as a minor factor in tylosin (244). It is the immediate biosynthetic precursor of tylosin and can be produced by a biosynthetically blocked mutant of S. fradiae (245—248). 6-Deoxy-2-0-methyl-D-allose [921-90-4/, a neutral sugar in macrocin, is a constituent of cardiac glycosides from Antiaris toxicaria and named javose (9, R = H, R/ = CH3) (249). 2 "-0-Demethylmacrocin (DOMM) obtained from another mutant of S. fradiae (246,250—252), is the penultimate precursor of tylosin (245—248). Other mutant strains of S. fradiae yield shunt metabolites of tylosin in which mycinose is altered (251,252). CP-56,064 [88378-53-4] is an analogue of tylosin in which mycarose is replaced by amicetose (5, R = H, R = OH) (253). 

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