DnrI of Streptomyces peucetius binds to the resistance genes, drrAB and drrC but is activated by daunorubicin

The master regulator, DnrI of Streptomyces peucetius is a member of the family of transcriptional activator, Streptomyces antibiotic regulatory proteins (SARP), which controls the biosynthesis of antitumor anthracycline, daunorubicin (DNR) and doxorubicin (DXR). The binding of DnrI to the heptameric repeat sequence found within the -35 promoter region of biosynthetic gene, dpsE activates it. To combat the increased level of intracellular DNR, the cell has developed self resistance mechanism mediated by drrAB and drrC genes which are regulated by regulatory genes. We find that a drug non-producing mutant, ΔdpsA, showed sensitive phenotype in plate assay along with an increased level of dnrI transcript. Whereas the mutant grown in the presence of DNR showed a resistant phenotype with a six and eight folds increase in drrAB and drrC transcripts respectively. Computational studies followed by molecular docking showed that DnrI bound as a monomer to a slightly modified heptameric DNA motif, 5'-ACACGCA in drrA and 5'-ACAACCT in drrC which was also proved by electrophoretic mobility shift assay. These findings confirm that DnrI belongs to winged helix-turn-helix DNA-binding protein with Tetratricopeptide Repeat domain. The transcriptional regulator DnrI binds to the resistance genes at specific sites but they are activated only when an increased load of intracellular DNR is sensed.

Daunorubicin efflux in Streptomyces peucetius modulates biosynthesis by feedback regulation

Streptomyces peucetius self-resistance genes drrA and drrB encode membrane-associated proteins that function like an ABC transporter for the efflux of daunorubicin and to maintain a constant subinhibitory physiological concentration of the drug within the cell. In this study, the drrA and drrB operons were disrupted for investigating drug production, self-resistance and regulation. The drrA-drrB null mutant was highly sensitive to daunorubicin. A 10-fold decrease in drug production was observed in the null mutant compared with the wild-type strain. We propose that the absence of a drug-specific efflux pump increases the intracellular concentration of daunorubicin, which is sensed by the organism to turn down drug production. Quantitative real-time PCR analysis of the mutant showed a drastic reduction in the expression of the key regulator dnrI and polyketide synthase gene dpsA. However, the expression of regulatory genes dnrO and dnrN was increased. Feedback regulation based on the intracellular daunorubicin concentration is discussed.

Natural-product sugar biosynthesis and enzymatic glycodiversification

Many biologically active small-molecule natural products produced by microorganisms derive their activities from sugar substituents. Changing the structures of these sugars can have a profound impact on the biological properties of the parent compounds. This realization has inspired attempts to derivatize the sugar moieties of these natural products through exploitation of the sugar biosynthetic machinery. This approach requires an understanding of the biosynthetic pathway of each target sugar and detailed mechanistic knowledge of the key enzymes. Scientists have begun to unravel the biosynthetic logic behind the assembly of many glycosylated natural products and have found that a core set of enzyme activities is mixed and matched to synthesize the diverse sugar structures observed in nature. Remarkably, many of these sugar biosynthetic enzymes and glycosyltransferases also exhibit relaxed substrate specificity. The promiscuity of these enzymes has prompted efforts to modify the sugar structures and alter the glycosylation patterns of natural products through metabolic pathway engineering and enzymatic glycodiversification. In applied biomedical research, these studies will enable the development of new glycosylation tools and generate novel glycoforms of secondary metabolites with useful biological activity.

In situ monitoring of streptothricin production by Streptomyces rochei F20 in soil and rhizosphere

The onset of streptothricin (ST) biosynthesis in Streptomyces rochei F20 was studied by using reverse transcription-PCR (RT-PCR) to detect transcripts of ST genes during growth in liquid medium, soil, and the rhizosphere. In situ results correlated with those obtained in vitro, illustrating the growth phase-dependent manner of ST production by F20. Maximal transcription of ST resistance (sttR) and biosynthesis (sttA) genes occurred during the transition between the exponential and stationary phases of growth, when the specific growth rate (micro) started to decline. A higher level of gene expression of sttR versus sttA was observed in all experiments. In liquid culture, maximal transcript accumulation of the sttA gene was only ca. 40% that of the sttR gene. sttA and sttR mRNAs were detected in soil containing approximately 10(6) CFU of growing cells g of soil(-1). sttR mRNA was detected in sterile and nonsterile rhizosphere colonized with growing mycelium of F20 at 1.2 x 10(6) and 4.0 x 10(5) CFU g of soil(-1), respectively. However, neither sttR nor sttA transcripts were detected by RT-PCR in the rhizoplane, which supported a lower population density of F20 than the rhizosphere.

Biosynthetic origin of hygromycin A

Hygromycin A, an antibiotic produced by Streptomyces hygroscopicus, is an inhibitor of bacterial ribosomal peptidyl transferase. The antibiotic binds to the ribosome in a distinct but overlapping manner with other antibiotics and offers a different template for generation of new agents effective against multidrug-resistant pathogens. Reported herein are the results from a series of stable-isotope-incorporation studies demonstrating the biosynthetic origins of the three distinct structural moieties which comprise hygromycin A. Incorporation of [1-(13)C]mannose and intact incorporation of D-[1,2-(13)C(2)]glucose into the 6-deoxy-5-keto-D-arabino-hexofuranose moiety are consistent with a pathway in which mannose is converted to an activated L-fucose, via a 4-keto-6-deoxy-D-mannose intermediate, with a subsequent unusual mutation of the pyranose to the corresponding furanose. The aminocyclitol moiety was labeled by D-[1,2-(13)C(2)]glucose in a manner consistent with formation of myo-inositol and a subsequent unprecedented oxidation and transamination of the C-2 hydroxyl group to generate neo-inosamine-2. Incorporation of [carboxy-(13)C]-4-hydroxybenzoic acid and intact incorporation of [2,3-(13)C(2)]propionate are consistent with a polyketide synthase-type decarboxylation condensation to generate the 3,4-dihydroxy-alpha-methylcinnamic acid moiety of hygromycin A. No labeling of hygromycin A was observed when [3-(13)C]tyrosine, [3-(13)C]phenylalanine, or [carboxy-(13)C]benzoic acid was used, suggesting that the 4-hydroxybenzoic acid is derived directly from chorismic acid. Consistent with this hypothesis was the observation that hygromycin A titers could be reduced by addition of N-(phosphonomethyl)-glycine (an inhibitor of chorismic acid biosynthesis) and restored by coaddition of 4-hydroxybenzoic acid. The convergent biosynthetic pathway established for hygromycin A offers significant versatility for applying the techniques of combinatorial and directed biosynthesis to production of new antibiotics which target the ribosomal peptidyl transferase activity.

Cloning and analysis of the spinosad biosynthetic gene cluster of Saccharopolyspora spinosa

BACKGROUND

Spinosad is a mixture of novel macrolide secondary metabolites produced by Saccharopolyspora spinosa. It is used in agriculture as a potent insect control agent with exceptional safety to non-target organisms. The cloning of the spinosyn biosynthetic gene cluster provides the starting materials for the molecular genetic manipulation of spinosad yields, and for the production of novel derivatives containing alterations in the polyketide core or in the attached sugars.

RESULTS

We cloned the spinosad biosynthetic genes by molecular probing, complementation of blocked mutants, and cosmid walking, and sequenced an 80 kb region. We carried out gene disruptions of some of the genes and analyzed the mutants for product formation and for the bioconversion of intermediates in the spinosyn pathway. The spinosyn gene cluster contains five large open reading frames that encode a multifunctional, multi-subunit type I polyketide synthase (PKS). The PKS cluster is flanked on one side by genes involved in the biosynthesis of the amino sugar forosamine, in O-methylations of rhamnose, in sugar attachment to the polyketide, and in polyketide cross-bridging. Genes involved in the early common steps in the biosynthesis of forosamine and rhamnose, and genes dedicated to rhamnose biosynthesis, were not located in the 80 kb cluster.

CONCLUSIONS

Most of the S. spinosa genes involved in spinosyn biosynthesis are found in one 74 kb cluster, though it does not contain all of the genes required for the essential deoxysugars. Characterization of the clustered genes suggests that the spinosyns are synthesized largely by mechanisms similar to those used to assemble complex macrolides in other actinomycetes. However, there are several unusual genes in the spinosyn cluster that could encode enzymes that generate the most striking structural feature of these compounds, a tetracyclic polyketide aglycone nucleus.

Genes for production of the enediyne antitumor antibiotic C-1027 in Streptomyces globisporus are clustered with the cagA gene that encodes the C-1027 apoprotein

C-1027, the most potent member of the enediyne antitumor antibiotic family, is produced by Streptomyces globisporus C-1027 and consists of an apoprotein (encoded by the cagA gene) and a nonpeptidic chromophore. The C-1027 chromophore could be viewed as being derived biosynthetically from a benzoxazolinate, a deoxyamino hexose, a beta-amino acid, and an enediyne core. By adopting a strategy for cloning of the C-1027 biosynthesis gene cluster by mapping a putative dNDP-glucose 4,6-dehydratase (NGDH) gene to cagA, we have localized 75 kb of contiguous DNA from S. globisporus. DNA sequence analysis of two regions of the cloned gene cluster revealed two genes, sgcA and sgcB, that encode an NGDH enzyme and a transmembrane efflux protein, respectively, and confirmed that the cagA gene resides approximately 14 kb upstream of the sgcAB locus. The involvement of the cloned gene cluster in C-1027 biosynthesis was demonstrated by disrupting the sgcA gene to generate C-1027-nonproducing mutants and by complementing the sgcA mutants in vivo to restore C-1027 production. These results represent the first cloning of a gene cluster for enediyne antitumor antibiotic biosynthesis and provide a starting point for future genetic and biochemical investigations of C-1027 biosynthesis.