Kozo Ochi

New strategies for drug discovery: activation of silent or weakly expressed microbial gene clusters.

Genome sequencing of Streptomyces, myxobacteria, and fungi showed that although each strain contains genes that encode the enzymes to synthesize a plethora of potential secondary metabolites, only a fraction are expressed during fermentation. Interest has therefore grown in the activation of these cryptic pathways. We review current progress on this topic, describing concepts for activating silent genes, utilization of “natural” mutant-type RNA polymerases and rare earth elements, and the applicability of ribosome engineering to myxobacteria and fungi, the microbial groups known as excellent searching sources, as well as actinomycetes, for secondary metabolites.

Activating the expression of bacterial cryptic genes by rpoB mutations in RNA polymerase or by rare earth elements

Since bacteria were found to contain genes encoding enzymes that synthesize a plethora of potential secondary metabolites, interest has grown in the activation of these cryptic pathways. Homologous and heterologous expression of these cryptic secondary metabolite-biosynthetic genes, often “silent” under ordinary laboratory fermentation conditions, may lead to the discovery of novel secondary metabolites. We review current progress on this topic, describing concepts for activating silent genes. We especially focus on genetic manipulation of transcription and translation, as well as the utilization of rare earth elements as a novel method to activate the silent genes. The possible roles of silent genes in bacterial physiology are also discussed.

Activation and Products of the Cryptic Secondary Metabolite Biosynthetic Gene Clusters by Rifampin Resistance (rpoB) Mutations in Actinomycetes

A subset of rifampin resistance (rpoB) mutations result in the overproduction of antibiotics in various actinomycetes, including Streptomyces, Saccharopolyspora, and Amycolatopsis, with H437Y and H437R rpoB mutations effective most frequently. Moreover, the rpoB mutations markedly activate (up to 70-fold at the transcriptional level) the cryptic/silent secondary metabolite biosynthetic gene clusters of these actinomycetes, which are not activated under general stressful conditions, with the exception of treatment with rare earth elements. Analysis of the metabolite profile demonstrated that the rpoB mutants produced many metabolites, which were not detected in the wild-type strains. This approach utilizing rifampin resistance mutations is characterized by its feasibility and potential scalability to high-throughput studies and would be useful to activate and to enhance the yields of metabolites for discovery and biochemical characterization.

Rare earth elements activate the secondary metabolite–biosynthetic gene clusters in Streptomyces coelicolor A3(2)

Genome sequencing projects have revealed many biosynthesis gene clusters for the production of as-yet unknown secondary metabolites, especially in actinomycetes. Here, we report that the rare earth elements, scandium and/or lanthanum, markedly activate, ranging from 2.5- to 12-fold, the expression of nine genes belonging to nine secondary metabolite–biosynthetic gene clusters of Streptomyces coelicolor A3(2) when added to the medium at low concentrations. HPLC analysis of ethyl acetate-extractable metabolites indicated the detectability of several compounds only in the rare earth-treated cultures. This approach should facilitate discovery of new biologically active compounds and the study of secondary metabolite production.

Scandium Stimulates the Production of Amylase and Bacilysin in Bacillus subtilis

ABSTRACT We investigated the effects of rare earth elements on enzyme production and secondary metabolism in Bacillus subtilis. Addition of scandium to the growth medium stimulated the production of both amylase and bacilysin at the transcriptional level, thus showing scandium to have a remarkable impact in B. subtilis.

Undecaprenyl Pyrophosphate Involvement in Susceptibility of Bacillus subtilis to Rare Earth Elements

The rare earth element scandium has weak antibacterial potency. We identified a mutation responsible for a scandium-resistant phenotype in Bacillus subtilis. This mutation was found within the uppS gene, which encodes undecaprenyl pyrophosphate synthase, and designated uppS86 (for the Thr-to-Ile amino acid substitution at residue 86 of undecaprenyl pyrophosphate synthase). The uppS86 mutation also gave rise to increased resistance to bacitracin, which prevents cell wall synthesis by inhibiting the dephosphorylation of undecaprenyl pyrophosphate, in addition to enhanced amylase production. Conversely, overexpression of the wild-type uppS gene resulted in increased susceptibilities to both scandium and bacitracin. Moreover, the mutant lacking undecaprenyl pyrophosphate phosphatase (BcrC) showed increased susceptibility to all rare earth elements tested. These results suggest that the accumulation of undecaprenyl pyrophosphate renders cells more susceptible to rare earth elements. The availability of undecaprenyl pyrophosphate may be an important determinant for susceptibility to rare earth elements, such as scandium.

Development of the ability to produce secondary metabolites in Streptomyces through the acquisition of erythromycin resistance

Development of the ability to produce secondary metabolites in Streptomyces through the acquisition of erythromycin resistance

Production improvement of antifungal, antitrypanosomal nucleoside sinefungin by rpoB mutation and optimization of resting cell system of Streptomyces incarnatus NRRL 8089

Sinefungin, a nucleoside antibiotic with potent antifungal, antiviral, and anti-trypanosome activities, has been a target for production enhancement in the past decades through medium optimization and strain improvement. For the purpose of introducing a more rational approach, we induced rpoB mutation in the producer strain, Streptomyces incarnatus NRRL 8089, by optimized UV-irradiation, and a resulting rifampicin-resistant strain rif-400 increased the sinefungin production by 7-fold. The growth and melanin production were obviously accelerated in the rifampicin-resistant high-producer mutant, while the morphological differentiation such as aerial mycelia and spiked-spore formation was retained. Molecular cloning and DNA sequencing identified a single mutation A1340G in the rpoB gene, which encodes the beta-subunit of RNA polymerase, and the resulting amino acid substitution Asp447Gly corresponded to one of mutations that reportedly allowed the transcriptional up-regulation of actinorhodin production in S. coelicolor A3(2). Sinefungin production was further enhanced by resting cell system using the rpoB mutant strain in the presence of 10 mM L-Arg. D-Arg or L-ornithine did not enhance the sinefungin production, and >50 mM urea strongly suppressed the nucleoside antibiotic production, supporting the proposed biosynthetic mechanism by which urea is liberated from the guanidino-group-bearing intermediate that is produced by enzymatic condensation of L-Arg and ATP.

A magic bullet for antibiotic discovery.

Activation/exploitation of biosynthetic pathways for useful metabolites is a major current interest. The metabolism remodeling approach developed by Craney and colleagues in this issue of (Chemistry & Biology), in which small molecule probes alter the secondary metabolites produced by streptomycetes, could lead to discovery of a multitude of novel antibiotics and other drugs.

The mthA Mutation Conferring Low-Level Resistance to Streptomycin Enhances Antibiotic Production in Bacillus subtilis by Increasing the S-Adenosylmethionine Pool Size

Certain Str(r) mutations that confer low-level streptomycin resistance result in the overproduction of antibiotics by Bacillus subtilis. Using comparative genome-sequencing analysis, we successfully identified this novel mutation in B. subtilis as being located in the mthA gene, which encodes S-adenosylhomocysteine/methylthioadenosine nucleosidase, an enzyme involved in the S-adenosylmethionine (SAM)-recycling pathways. Transformation experiments showed that this mthA mutation was responsible for the acquisition of low-level streptomycin resistance and overproduction of bacilysin. The mthA mutant had an elevated level of intracellular SAM, apparently acquired by arresting SAM-recycling pathways. This increase in the SAM level was directly responsible for bacilysin overproduction, as confirmed by forced expression of the metK gene encoding SAM synthetase. The mthA mutation fully exerted its effect on antibiotic overproduction in the genetic background of rel(+) but not the rel mutant, as demonstrated using an mthA relA double mutant. Strikingly, the mthA mutation activated, at the transcription level, even the dormant ability to produce another antibiotic, neotrehalosadiamine, at concentrations of 150 to 200 μg/ml, an antibiotic not produced (<1 μg/ml) by the wild-type strain. These findings establish the significance of SAM in initiating bacterial secondary metabolism. They also suggest a feasible methodology to enhance or activate antibiotic production, by introducing either the rsmG mutation to Streptomyces or the mthA mutation to eubacteria, since many eubacteria have mthA homologues.