Intergeneric conjugal gene transfer fromEscherichia colito the sweet potato pathogenStreptomyces ipomoeae

Comparative genomics of the niche-specific plant pathogen Streptomyces ipomoeae reveal novel genome content and organization

IMPORTANCE

While most plant-pathogenic Streptomyces species cause scab disease on a variety of plant hosts, Streptomyces ipomoeae is the sole causative agent of soil rot disease of sweet potato and closely related plant species. Here, genome sequencing of virulent and avirulent S. ipomoeae strains coupled with comparative genomic analyses has identified genome content and organization features unique to this streptomycete plant pathogen. The results here will enable future research into the mechanisms used by S. ipomoeae to cause disease and to persist in its niche environment.

Discovery, Yield Improvement, and Application in Marine Coatings of Potent Antifouling Compounds Albofungins Targeting Multiple Fouling Organisms

Marine biofouling caused huge economic losses of maritime industries. We aim to develop high-efficient, less-toxic, and cost-effective antifoulants to solve the problems of biofouling. In this study, we described the antifouling compounds albofungin and its derivatives (albofungin A, chrestoxanthone A, and chloroalbofungin) isolated from the metabolites of bacterium Streptomyces chrestomyceticus BCC 24770, the construction of high-yield strains for albofungin production, and application of albofungin-based antifouling coatings. Results showed that these albofungins have potent antibiofilm activities against Gram-positive and Gram-negative bacteria and anti-macrofouling activities against larval settlement of major fouling organisms with low cytotoxicity. With the best antifouling activity and highest yield in bacterial culture, albofungin was subsequently incorporated with hydrolyzable and degradable copolymer to form antifouling coatings, which altered biofilm structures and prevented the settlement of macrofouling organisms in marine environments. Our results suggested that albofungins were promising antifouling compounds with potential application in marine environments.

Stepwise increase of thaxtomins production in Streptomyces albidoflavus J1074 through combinatorial metabolic engineering

Herbicide-resistance in weeds has become a serious threat to agriculture across the world. Thus, there is an urgent need for the discovery and development of herbicides with new modes of action. Thaxtomin phytotoxins are a group of nitrated diketopiperazines produced by potato common scab-causing phytopathogen Streptomyces scabies and other actinobacterial pathogens. They are generally considered to function as inhibitors of cellulose synthesis in plants, and thus have great potential to be used as natural herbicides. Generation of an overproducing strain is crucial for the scale-up production of thaxtomins and their wide use in agriculture. In the present study, we employed a stepwise strategy by combining heterologous expression, repressor deletion, activator overexpression, and optimization of fermentation media for high-level production of thaxtomins. The maximum yield of 728 mg/L thaxtomins was achieved with engineered Streptomyces albidoflavus J1074 strains in shake-flask cultures, and it was approximately 36-fold higher than S. albidoflavus J1074 carrying the unmodified cluster. Moreover, the yield of thaxtomins could reach 1973 mg/L when the engineered strain was cultivated in a small-scale stirred-tank bioreactor. This is the highest titer reported to date, representing a significant leap forward for the scale-up production of thaxtomins. Our study presents a robust, easy-to-use system that will be broadly useful for improving titers of bioactive compounds in many Streptomyces species.

An Efficient Method To Generate Gene Deletion Mutants of the Rapamycin-Producing Bacterium Streptomyces iranensis HM 35

Streptomyces iranensis HM 35 is an alternative rapamycin producer to Streptomyces rapamycinicus Targeted genetic modification of rapamycin-producing actinomycetes is a powerful tool for the directed production of rapamycin derivatives, and it has also revealed some key features of the molecular biology of rapamycin formation in S. rapamycinicus. The approach depends upon efficient conjugational plasmid transfer from Escherichia coli to Streptomyces, and the failure of this step has frustrated its application to Streptomyces iranensis HM 35. Here, by systematically optimizing the process of conjugational plasmid transfer, including screening of various media, and by defining optimal temperatures and concentrations of antibiotics and Ca(2+) ions in the conjugation media, we have achieved exconjugant formation for each of a series of gene deletions in S. iranensis HM 35. Among them were rapK, which generates the starter unit for rapamycin biosynthesis, and hutF, encoding a histidine catabolizing enzyme. The protocol that we have developed may allow efficient generation of targeted gene knockout mutants of Streptomyces species that are genetically difficult to manipulate.

IMPORTANCE

The developed protocol of conjugational plasmid transfer from Escherichia coli to Streptomyces iranensis may allow efficient generation of targeted gene knockout mutants of other genetically difficult to manipulate, but valuable, Streptomyces species.

Development of an intergeneric conjugal transfer system for xinaomycins-producing Streptomyces noursei Xinao-4

To introduce DNA into Streptomyces noursei xinao-4, which produces xinaomycins, we explored an intergeneric conjugal transfer system. High efficiency of conjugation (8 × 10⁻³ exconjugants per recipient) was obtained when spores of S. noursei xinao-4 were heat-shocked at 50 °C for 10 min, mixed with Escherichia coli ET12567 (pUZ8002/pSET152) in the ratio of 1:100, plated on 2CMY medium containing 40 mmol/L MgCl₂, and incubated at 30 °C for 22 h. With this protocol, the plasmids pKC1139 and pSET152 were successfully transferred from E. coli ET12567 (pUZ8002) with different frequencies. Among all parameters, the ratio of donor to recipient cell number had the strongest effect on the transformation efficiency. In order to validate the above intergeneric conjugal transfer system, a glycosyltransferase gene was cloned and efficiently knocked out in S. noursei xinao-4 using pSG5-based plasmid pKC1139.

An efficient intergeneric conjugation of DNA from Escherichia coli to mycelia of the lincomycin-producer Streptomyces lincolnensis

Streptomyces lincolnensis is a producer of lincomycin, which is a lincosamide antibiotic for the treatment of infective diseases caused by Gram-positive bacteria. S. lincolnensis is refractory to introducing plasmid DNA into cells because of resistance of foreign DNAs and poor sporulation. In this study, a simple and efficient method of transferring plasmids into S. lincolnensis through the intergeneric Escherichia coli-mycelia conjugation was established and optimized for the first time. The recipient mycelia of S. lincolnensis were prepared in liquid SM medium containing 10.3% sucrose for three days. The dispersed mycelia were conjugated with competent E. coli donor cells. The exconjugants were regenerated efficiently on solid mannitol soya flour (MS) medium containing 20 mM MgCl₂. The average conjugation frequency was observed at 1.1 × 10⁻⁴ per input donor cell and validated functionally by transferring two types of vectors containing lincomycin resistance genes lmrA, lmrB and lmrC into S. lincolnensis mycelia. The data of fermentation in shaking flasks showed the lincomycin yield of the exconjugants increased by 52.9% for the multiple copy vector and 38.3% for the integrative one, compared with the parental strain. The efficient and convenient method of intergeneric E. coli-mycelia conjugation in this study provides a promising procedure to introduce plasmid DNA into other refractory streptomycetes.

Evidence that thaxtomin C is a pathogenicity determinant of Streptomyces ipomoeae, the causative agent of Streptomyces soil rot disease of sweet potato

Streptomyces ipomoeae is the causal agent of Streptomyces soil rot of sweet potato, a disease marked by highly necrotic destruction of adventitious roots, including the development of necrotic lesions on the fleshy storage roots. Streptomyces potato scab pathogens produce a phytotoxin (thaxtomin A) that appears to facilitate their entrance into host plants. S. ipomoeae produces a less-modified thaxtomin derivative (thaxtomin C) whose role in pathogenicity has not been examined. Here, we cloned and sequenced the thaxtomin gene cluster (txt) of S. ipomoeae, and we then constructed targeted txt mutants that no longer produced thaxtomin C. The mutants were unable to penetrate intact adventitious roots but still caused necrosis on storage-root tissue. These results, taken in context with previous histopathological study of S. ipomoeae infection, suggest that thaxtomin C plays an essential role in inter- and intracellular penetration of adventitious sweet potato roots by S. ipomoeae. Once inside the plant host, the pathogen uses one or more yet-to-be-determined factors to necrotize root tissue, including that of any storage roots it encounters.