Regulatory perspective of antibiotic biosynthesis in Streptomyces

Trends in the two-component system's role in the synthesis of antibiotics by Streptomyces

Despite the advances in understanding the regulatory networks for secondary metabolite production in Streptomyces, the participation of the two-component systems (TCS) in this process still requires better characterization. These sensing systems and their responses to environmental stimuli have been described by evaluating mutant strains with techniques that allow in-depth regulatory responses. However, defining the stimulus that triggers their activation is still a task. The transmembrane nature of the sensor kinases and the high content of GC in the streptomycetes represent significant challenges in their study. In some examples, adding elements to the assay medium has determined the respective ligand. However, a complete TCS description and characterization requires specific amounts of the involved proteins that are most difficult to obtain. The availability of enough sensor histidine kinase concentrations could facilitate the identification of the ligand-protein interaction, and besides would allow the establishment of its phosphorylation mechanisms and determine their tridimensional structure. Similarly, the advances in the development of bioinformatics tools and novel experimental techniques also promise to accelerate the TCSs description and provide knowledge on their participation in the regulation processes of secondary metabolite formation. This review aims to summarize the recent advances in the study of TCSs involved in antibiotic biosynthesis and to discuss alternatives to continue their characterization. KEY POINTS: • TCSs are the environmental signal transducers more abundant in nature. • The Streptomyces have some of the highest number of TCSs found in bacteria. • The study of signal transduction between SHKs and RRs domains is a big challenge.

Effects of S-adenosylmethionine on production of secondary metabolites in Streptomycesdiastatochromogenes 1628

Streptomyces are famous for their ability to synthesize a large number of bioactive compounds as secondary metabolites containing antibiotics, enzyme inhibitors, and other small molecules with potential physiological activity (Niu et al., 2016; Song et al., 2019; Yin et al., 2019). Secondary metabolites are produced by a multi-step reaction of a primary metabolite as a precursor (Liu et al., 2013; Li et al., 2021). Therefore, it is of great research significance to increase the overall synthesis level of antibiotics by increasing the amount of synthesis of precursors.

Bioconversion of potato solid waste into antifungals and biopigments using Streptomyces spp

Potato waste was processed and used as a sole substrate for simultaneously producing antifungals and biopigments using Streptomyces spp. Out of three different Streptomyces isolates, strain SO6 stood out due to its ability to produce antifungals against economically important fungal phytopathogens and intracellular biopigments using potato waste powders without additional nutrients. This strain also showed the potential to secrete a broad range of enzymes for fermentation of eight sugars that could be involved in potato waste bioconversion. The results of the fermentation assay indicated that Streptomyces sp. strain SO6 degrades potato wastes during submerged fermentation, diminishing total dry weight and increasing reducing sugars from 0.3 to 3.6 mg·mL⁻¹ and total proteins from 70.6 to 187.7 μg·mL⁻¹. The results showed that Streptomyces strain SO6 was able to convert the potato waste into 0.96 mg·g⁻¹ of diffusible antifungals and 1.75 mg·g⁻¹ of reddish-purple biopigments. On the contrary, an absence of pigment production was observed during the fermentation of the commercial medium used as reference. According to our results, replacement of commercial culture media with available low-cost agroindustrial wastes for producing bioactive chemicals is a real opportunity to enhance the Streptomyces pigment production and antibiotic sustainability with cost-competitiveness. To our knowledge, this is the first report on the simultaneous production of biopigments and diffusible antifungal antibiotics produced by Streptomyces spp. using potato solid waste as the sole nutrient source.

Molecular mechanism of mureidomycin biosynthesis activated by introduction of an exogenous regulatory gene ssaA into Streptomyces roseosporus

Mureidomycins (MRDs), a group of unique uridyl-peptide antibiotics, exhibit antibacterial activity against the highly refractory pathogen Pseudomonas aeruginosa. Our previous study showed that the cryptic MRD biosynthetic gene cluster (BGC) mrd in Streptomyces roseosporus NRRL 15998 could not be activated by its endogenous regulator 02995 but activated by an exogenous activator SsaA from sansanmycin’s BGC ssa of Streptomyces sp. strain SS. Here we report the molecular mechanism for this inexplicable regulation. EMSAs and footprinting experiments revealed that SsaA could directly bind to a 14-nt palindrome sequence of 5′-CTGRCNNNNGTCAG-3′ within six promoter regions of mrd. Disruption of three representative target genes (SSGG-02981, SSGG-02987 and SSGG-02994) showed that the target genes directly controlled by SsaA were essential for MRD production. The regulatory function was further investigated by replacing six regions of SSGG-02995 with those of ssaA. Surprisingly, only the replacement of 343–450 nt fragment encoding the 115–150 amino acids (AA) of SsaA could activate MRD biosynthesis. Further bioinformatics analysis showed that the 115–150 AA situated between two conserved domains of SsaA. Our findings significantly demonstrate that constitutive expression of a homologous exogenous regulatory gene is an effective strategy to awaken cryptic biosynthetic pathways in Streptomyces.

The regulatory cascades of antibiotic production in Streptomyces

Streptomyces is famous for its capability to produce the most abundant antibiotics in all kingdoms. All Streptomyces antibiotics are natural products, whose biosynthesis from the so-called gene clusters are elaborately regulated by pyramidal transcriptional regulatory cascades. In the past decades, scientists have striven to unveil the regulatory mechanisms involved in antibiotic production in Streptomyces. Here we mainly focus on three aspects of the regulation on antibiotic production. 1. The onset of antibiotic production triggered by hormones and their coupled receptors as regulators; 2. The cascades of global and pathway-specific regulators governing antibiotic production; 3. The feedback regulation of antibiotics and/or intermediates on the gene cluster expression for their coordinated production. This review will summarize how the antibiotic production is stringently regulated in Streptomyces based on the signaling, and lay a theoretical foundation for improvement of antibiotic production and potentially drug discovery.

Heterologous production of chlortetracycline in an industrial grade Streptomyces rimosus host

High-yielding industrial Streptomyces producer is usually obtained by multiple rounds of random mutagenesis and screening. These strains have great potential to be developed as the versatile chassis for the discovery and titer improvement of desired heterologous products. Here, the industrial strain Streptomyces rimosus 461, which is a high producer of oxytetracycline, has been engineered as a robust host for heterologous expression of chlortetracycline (CTC) biosynthetic gene cluster. First, the industrial chassis strain SR0 was constructed by deleting the whole oxytetracycline gene cluster of S. rimosus 461. Then, the biosynthetic gene cluster ctc of Streptomyces aureofaciens ATCC 10762 was integrated into the chromosome of SR0. With an additional constitutively expressed cluster-situated activator gene ctcB, the CTC titer of the engineering strain SRC1 immediately reached 1.51 g/L in shaking flask. Then, the CTC titers were upgraded to 2.15 and 3.27 g/L, respectively, in the engineering strains SRC2 and SRC3 with the enhanced ctcB expression. Further, two cluster-situated resistance genes were co-overexpressed with ctcB. The resultant strain produced CTC up to 3.80 g/L in shaking flask fermentation, which represents 38 times increase in comparison with that of the original producer. Overall, SR0 presented in this study have great potential to be used for heterologous production of tetracyclines and other type II polyketides.